How long does it take for one to remember a previous life in a sitting?
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.