Pressurized aircraft often leak like sieves once they get older. It's difficult to set limits unless an OEM provides a specific procedure, and an OEM procedure may be designed for new a/c as a quality control measure without any latitude for service deterioration.
There isn't really any guidance on leak rates in the FARs for cert purposes. There is only language limiting exposure time for passengers to low cabin pressures for emergency descents, which has an indirect impact on leak rates. Beyond that it's just the ability of the bleed sources to keep the cabin pressurized.
From an operational standpoint, a leak will get looked at either when there are pressurization irregularities like a crew writing up a "slow to pressurize" snag of some kind, or when there are complaints about noise or drafts from door seals.
An airline may do a fuselage leak check on a heavy check interval like C Check (typically 5000 hrs) where they will pressurize the fuselage in the hangar (or use a vacuum system as ymb1 mentioned) and go around listening and spraying with a soap bottle. It'll depend on their maintenance program. If there are no service limits for leak rates however, they probably wouldn't do that kind of test if there are no pass/fail limits and will only do it as a troubleshooting exercise.