If you're asking about Portulaca grandiflora &/or umbraticola, each flower lasts for 1 day. The inflorescence can produce new flowers for many days. When I see a stem without any more unopened buds, I cut them off.
This pic of P. umbraticola shows yesterday's shriveled petals, seed pods forming where older flowers were, and new buds that will open today or tomorrow.
When this whole inflorescence is finished, I will cut it off. Shriveled P. grandiflora petals do seem more apt to get stuck on the foliage than umbraticola. If it makes you feel better to remove them, go for it, but it won't stop seeds from forming unless you remove the little pod that's left or cut off the whole inflorescence when you see there are no more buds forming there.
