There should never be a given number of days set in stone for this.
The days between your last shot and you begining your PCT will depend on
1) The AAS
2) The dosage
3) The lenght of time
Let look at examples.
look at
http://www.roidcalc.com/
(500mg done all at once on the first monday of the week, uncheck view active, we want to know how much in lingering in the depots)
500 mg EW of test enant on a 8 week cycle...
At the begining of week
11 you have 42mg left over, that is low enough to start.
1500mg EW 8 week also...
At the begining of week
12 you have 47mg left over, that is low enough to start.
Ofcourse if you were using Deca at high dosages it would be week 13.
Also as I menioned the lenght of time for the cycle. A 4 week cycle for example will have much less depot build ups than a 12 week.