How often do you check your snails to make sure that they are alive?

Since snails can pass randomly even when they are fully healthy (just due to old age), how often do you do a snail check to ensure that they have moved (and are alive)?

I personally will try to just check on them once a week or bi-weekly (with every species, to make sure that they are getting enough food, not stuck somewhere since I've had that happen before), but won't truly go on a search by moving things around if I don't see them for a full month or so.