Minimum transaction fee to COMPLETE when mempool is +50 MB As of June 11, 2017: 

Mempool History  - &lt; Blockchain.info


the mempool is currently about 50 MB, and  
a week ago, the mempool was about 100 MB.  
The blocks mined per day are about 150, so at 1 MB / block = 150 MB / day of data is added.


This tells me that, if demand for transactions decreases as it has for the last week, from a mempool of ~100MB, it may reach a mempool of zero within a week from today's ~50 MB.
-- Note that the mempool hasn't approached zero since April 16 this year.


Once it reaches zero, I'm assuming that even if the fee is set to zero, transactions will get confirmed "soon", and definitely within a day.  Yes?
If I'm moving .001 (= $3 US) and specify 3 satoshi as a fee, at what point in time, and is is likely that it will be confirmed at all, when the mempool is at 50 MB?   


I know that it increases probability as it ages.  However many miners won't touch transactions at all, unless there's a fee attached, OR all the fee-based transactions have been satisfied.  Correct?

So, what is PROBABLE TIME with the following mempool sizes would a $3 value / .001 transaction be confirmed with a 3 satoshi fee? 


50 MB mempool -- =  how many days or hours?
25 MB   "
10 MB   "
5  MB   "
I know that it increases probability as it ages. However many miners won't touch transactions at all, unless there's a fee attached, OR all the fee-based transactions have been satisfied. Correct?


As far as I can tell the system of giving older coins priority is no longer in use.

If we assume that miners will always take the 1MB with the highest fee rate out of their mempool (which is likely true except for their own transactions and accelerator activity), the only interesting question is:

What fee do I need to add to a transaction in order for it to be among the top 1MB for any of the next X blocks when I want it to confirm within X/6 hours.

Notably, the number of transactions in the mempool is not predictive in regard to this question. Rather interesting are:


lowest "natural" fee-rate in the past blocks
fee rates in the top X MB of the mempool
expected influx of competing transactions



  So, what is PROBABLE TIME with the following mempool sizes would a $3 value / .001 transaction be confirmed with a 3 satoshi fee? 


Since that is even below the minimum fee of 1,000 satoshi per kB, it will never confirm.

Update: I was wrong in the last sentence. This past weekend transactions with zero fee were confirmed. So, let me correct 'never' to 'once in a blue moon'.