if its the house plant then any time. preferably when not in flower, though.

if you look at it you will see there appear to be 'clusters' of foliage. knock the plant out of the pot and gently start to prise the clusters apart. they will naturally come apart where there are natural divisions.
then pot each section up into fresh compost, water well and wait for new growth.

SBG I really must do mine.

Been meaning do it for some time.

did mine last November when I was off work waiting for the all clear from the hospital. Kept me occupied for an afternoon.

I've done 'em any time, but the recommended time is spring, so now, really. (Assuming you mean Spathiphyllum)