Quite a lot of fisheries supply l/js these days and require you to wear theirs instead of your own. Under those circumstances I always check them over and also look for the last service date which should be written on the label inside the jacket. If it isn't there or the date is more than a year old I refuse to wear the l/j and demand another one. My view is that they require me to wear their l/js so therefore they have a duty of care to provide me with serviceable equipment.
Service intervals should be based on use - I would suggest that on a fishery where the lifejackets get daily use from different individuals that service intervals should be based on risk and be a maximum of every 3 months