I carry a pw (46), aw (48) gw (50) and sw (54). I have a lw (58) but don't put it in the bag that often.
I coach boy & girls high school golf. My equipment suggestions for my players is always contingent on their skill level. For example, a player who shoots high 80s - low 90s shouldn't carry a lob wedge in my opinion. He/she needs to gain confidence with a standard pw and sw at various yardages before adding a different tool (i.e. lw) to the bag.
As for how often to replace equipment, I generally default to less frequent is better.
I'm a 3 handicap and I've had my 50 and 54 wedges for over 3 years.