Short answer:

Yes, in general it is safe to assume that an L5-30R will supply you up to 30A (nominal, 24A continuous) at ~120V. Give or take.

If you're still reading...

Suppose you come across a L5-30R (120V, 30A receptacle) somewhere. By spec, can you assume that there will be 30 amps at it?

The ratings of these outlets tells you about the safety limits of the outlet, not the actual characteristics of the supply.

Assuming that the supply is correctly configured - correct cabling, circuit breakers and so forth - then you can reasonably expect to be able to draw 30A from that outlet.

However there are a few factors that can limit your actual available current, most notably the impedance of the circuit from your outlet back to the point of supply.

The biggest impediment (if you will pardon the pun) to your maximum current draw may be the impedance of the supply circuit. The higher the impedance of the supply circuit the larger the...