It has really depended on the title with a combination how many copies were allotted. With the lower print numbers, they sold out very quick and you needed to be there when sales opened. It also depends on which platform you are looking to buy for. Vita has been selling out more quickly.
With Oddworld and Octodad, which have had higher print runs, theyve lasted at least a day. Octodad is two days now (with only the PS4 version).
The best thing to do is be there when the sales open up to give your best chance for a copy. And even then, if its a very popular title/low print run, there have been some people who were there at opening and didnt get a copy. But keep in mind that LRG has been doing two batches. First batch usually at 10 a.m. EST and the other at 6 p.m. So if you miss the first one, and they sell out, then you can try and get your copy at the second batch. And its normally been the case that the 2nd batch doesnt sell out quite as fast.
Thanks for the info. I will definitely be awake at 10am and 6pm so thats good.