The duration of ico is up to team, they could be a lot of factors which they take in consideration.
I think the most influence on the ICO time is their confidence on if they will get to their target amount.

IF you must invest in the time of ICO, the starting phases are the best because then the most bonuses are but the min amount is also the highest.

I would recommend to invest after the ICO ends, wait for some time when the initial dip of token ends.