If you read all the spund valve links i posted there are 3 common PRV valves that people use. I've seen people report leaks on all of them.
Mine isn't quite that bad but its not ok to leave it attached unless there is active fermentation. My leakage problem got much better when I switched from tape to dope. The tape works ok for water but air/co2 has less surface tension than water so it can make its way out much easier.
It's "safe" to remove when the amount of remaining sugars will not generate an excessive pressure, or excessive carbonation in the beer if left unvented. This number is highly variable depending upon temperature and pressure, but it's on the order of 2-6 specific gravity points usually.
For ales i check it after a few days and then monitor as needed. The pressurizing curve is like an exponential approaching steady state (i think there's a better name for this but it's not coming to me), so you'll see your sharpest rise in the first few days, then it'll taper off. Check it frequently to start, then less and less so as its needed loss often.
For lagers i normally wont' even check until it's been sealed up for a week or two. They are much slower.
The further they are from the target pressure, the less frequently i check them. The closer they are, the more frequently i check until they reach steady state.
To check i just attach my spund valve with the pressure relief turned up high so it doesn't release. The gauge will read the pressure in the keg. I then pull the PRV on the keg to relieve.
