How often to monitor change in factors ?

How often do we need to monitor the change in factors ? Is 1 month sufficient enough ? If using 1M , do we just compare the backtest of the various factors over a 1M period or over a longer period but at 1 month frequency ?  Appreciate any advise or comments please.

No hard and fast rule for this. Really depends on your personal preference. The factors are quite stable so you should not need to monitor them every month. Perhaps once every 3 to 6 months would be good enough. You should use a longer period at a monthly frequency because a 1 month period is too short to confidently say that one factor is outperforming another factor.