[x] it depends
this one is about demographic, principle is the same http://www.conceptstew.co.uk/PAGES/nsamplesize.html
To judge strategy time wise its depend on the Strategy factors i.e :
1 - If the Strategy specified Pairs, Working Hours and Money Management, hence the results will depend on Risk/Reward % & Drawdown% which in my opinion take 2-3 months.
2 - If the Strategy NOT specified pairs, working hours and Money Management, hence you need to test it to get the most pairs protofilio and good Money Management and Low Risk/Reward% & Low Drawdown% which in my opinion will take on month more plus (2-3 months regular test).