At 4 months, as needed and either before or after a feeding. Always gets a clean, dry diaper before I lay him down to bed.
I still put vaseline on his bum before bed or when I am going somewhere and think he might be in his wet diaper for longer than about 3 hours. Makes me feel like his skin is more protected from the wetness and helps to prevent a rash, especially when his skin is just starting to look a little red, diaper rash cream for 2 or 3 changes, then vaseline to pretect the skin while it recovers.
Most times when I change him, I let him have about 10 minutes of diaperless time, to let his skin dry out really well after the really wet diapers.