How can one tell when it is time to replace the recoil spring?

Usually 3-5000rds seems to be a general rule of thumb. I usually wait till I see the gun ejecting emptys further than normal (8+ feet).

My S&W state LE rep told me the M&P pistols require the recoil spring to be replaced every 5,000 rounds fired. Replacing recoil springs at the manufacturer's recommended intervals is cheap insurance against pistol damage. Bill

I put at least 5K rounds thru each of my pistols a year (I only own a few), so I change every year. That's about right.

handgunner356 makes an excellent point about ejection pattern. It's a very good indicator.

I'll also add (not relevant to recoil springs) that if you have a normally good ejection pattern and one or two that fly off, it's usually a change in grip while shooting. I've seen it very often at our range.

Dave