What are the time demands on a PhD student who is helping set up a lab?
From what I have seen in general for molecular biology labs, there is generally a risk. Some of this risk is connected to the PI and some of it is connected to factors outside their control. I am in my 7th year right now and finishing up but my lab just moved across the country to set up a new lab. While this does not affect me, we have rotation students and I can see the impact it is having on them.


  
  What constitutes as normal and what is a red flag for PIs requiring
  start up assistance?
  


This will vary based on the lab's research needs. You can expect to be involved in a lot of organization/inventory, equipment setup, purchasing, account setup, and above all, a lot of waiting around. If the PI has a lot of specific needs within the building, then there may be specialized construction that has to take place. This could take week to months - both for the actual construction as well as general red tape and unforeseen delays. 

In general, you should know that your research will almost certainly be delayed. However, there are other ways to use your time that I elaborate below.

For red flags, I would say that you should try to determine the funding situation of the lab and what grants are active. There is no point waiting around if you have to wait even longer afterwards to get started on funding to come through. If you notice that the PI has had the lab space for 3-6 months and still has no progress, that could be a major red flag. On the other hand, the PI could be using that time very effectively to write grants and make connections. This is very situational.


  
  Are there benefits to being involved heavily with lab setup if you are
  not really committed to the academia route?
  


This depends on what you chose to do instead if you are not following the academia route. If you work in industry - say R&amp;D or testing for a major company or a career technician, many of these skills may be of use to you. Other potential benefits could relate more to your PI and future research. For example, a startup lab may have a new, enthusiastic professor exploring new areas of research. This can be an exciting environment to be in. There may be new students to change up the dynamic. It might be a great environment to bond with colleagues and so forth. This depends highly on the PI in question.

As NMJD pointed out in the comments, many of the tasks you will be involved in during lab set up could be very useful if you ever decide to stay in academia. You will be ordering equipment, setting up new technology, and budgeting for most of your academic career. This experience could also serve to prepare you if you ever start a lab of your own.


  
  What generates the time sink that seems to overburden most students?
  


A lot of the burden comes from waiting time (which isn't exactly a time sink). Setting up labs take a while but it isn't physical labor or time involved that takes the longest. Much of it comes from processing, financial constraint, and red tape.

I would be far more worried about these kinds of issues that may happen after setup: 


Delays in setting up animal facilities/breeding pairs or animal
lines.  
Cell culture contamination problems in a new building/room. 
Weird technical variability in experiments.
Getting used to new department/building procedures
Filling the responsibilities of departed graduate students that took care of the day-to-day in the lab
Lack of mentorship or guidance



  
  How can you mitigate this impact to time to degree?
  


Depending on the nature of the setup and how far along the lab is, you may be able to use this time to become deeply familiar with the literature in this field or participate in grant writing. This would be excellent for your training  and is very important. Take some statistics classes - it is far more important than most molecular biologists will have you believe!

Get to know people. Get to know the field and co-workers. Go have drinks with other labs and learn about their research. Find collaboration opportunities. If your program requires you to be a TA, get that out of the way early on. Focus on your classes. 

You can use this time to either prepare for the lab post-setup or get things out of the way that would interfere with your research later.


  
  Where does one draw the line between beneficial experience vs being taken advantage of?
  


Ultimately, I think you really need to talk to students in the lab or those nearby to get a better sense of what it's like "from the inside". You can only glean so much from the outsider perspective. Just know that in most cases you are taking a risk. But you can offset this by using your time wisely. If it's research you really want to do, it may be worth the risk.

If the trade-off between the time spent waiting and working through the bumpy starts are not outweighed by the research opportunity that could be provided then it is not worth it. If you find yourself wasting time, doing menial labor, and see no end in sight for the start up it's not worth it. 

You have to decide if it's worth it and it can be difficult without already being there. 

Good luck!