When in Ampicillin degraded (gone) in liquid TB-media? Concerns about selectivity
rather interesting question. Selection pressure is very important for any type of work. And it is true, amplicilin will degrade in media due to the secretion of beta-lactumase. This is a limitation that we must face, though there are ways of getting around this. 

For example, when cloning, on a plate sometime you will notice "smaller" colonies growing around a central large one. This is due to beta-lactumases. They are not a problem if you are going to take the central colony and place it into fresh media with new amplicilin. They will simply die out. No harm done. 

In liquid culture it is good to not "oversaturate the culture". Usually, do not let it get over OD600 = 1. In addition, do not use old stocks, as this causes problems with selection and use higher concentrations of ampicillin if you are still having troubles selecting! 

Generally speaking, 100ug/ml is standard for most selection with cloning, and should be sufficient for what you are doing. It seems that your protocol should work fine for protein isolation.

Last remarks -- it is a problem that most microbiologist do not think of. Ampicillin has its limits!

https://www.addgene.org/mol-bio-reference/antibiotics/

https://bitesizebio.com/10188/whats-the-problem-with-ampicillin-selection/