You want to have your total testosterone level at least down to a natty level. about 500 ng/dl give or take before starting with SERMs. Dr Scally, an expert in the field of restarting HPTA after steroid use is convinced that starting PCT too soon is one of the primary reasons for a failed PCT.
I've seen a couple guys post bloods at the 2 week mark, and they were pretty close to the 500 level after a standard test cycle. 2 to 3 weeks is the norm, with 3 weeks being a little safer as far as being sure your levels are down.
Dr Scally suggests 3 to 4 weeks, fwiw
With half lives there is something called the rule of fives, that it takes 5 half lives to reach a level that is near zero.... but zero is never reachable in theory.