Recovering spent cartridges

[Mahatatain]

What rules to other GMs use for recovering spent cartridges?

I've always assumed that if you have time to look for your spent cartridges that it should be relatively easy to find them (depending on what kind of terrain your firing from). However I fired an AK-47 for the first time at the weekend and I was very surprised at how far the ejecting bullet casing flew and I've now realised that recovering spent cartridges isn't as easy as I first thought, particularly from an assault rifle.

What guidelines do people use?