How long does it take for one to remember a previous life in a sitting? I have always wondered how long does it take for one to remember a past life in a sitting, an immediate previous life for example, does it take just a few minutes, or hours or maybe days? 

This answer briefed the following way to remember past lives:


Obtain the four rūpa-jhānas.
Exit the fourth jhāna and remember the last thing one did before one sat down.
Remember the last thing before the thing remembered in the last step. If at any time, one is unable to remember a given experience, one should revert to step one, and after leaving the fourth jhāna attempt to continue where one left off.
Repeat step 3 until one is able to remember the first moment of conception.
Attempt to remember the last moment before conception (i.e. the death moment).


Look at step 3, you have to remember the previous thing, and previous, and previous... Wow, how long would it take? Do we need to skip some huge steps (maybe 10 years of one's childhood) ? Or maybe the mind work lightningly faster when you are in that recollecting mode?
Personally I do not have any experience (I also like to have it though). Anyway according to my understanding the most difficult part is not step 3, it is step 1. Obtain the four rūpa-jhānas. If you can do it, I think the 3rd step is relatively easy compared to the 1st one. 

If you somehow managed to go back to your previous life, then going backward from there is considered as relatively easy. Moving this life to previous one is the difficult task (As I have heard). 

I heard several "Dhamma" talks around this topic and according to them if you are a good meditator and if you can practice the meditation most of the time, it is suggested a couple of years (it pretty much depends on you and your past Kamma).

Further, generally Buddhist meditators do not practice meditation focusing on getting these abilities.