I'd say Radiums probably could go 6 months without a sweat, after that... eh. However if you have a magnetic ballast they'll most likely keep the same intensity, they will however shift colors, all at the expense of pulling more juice. Unsure what you mean by "acting up some" I guess you mean algae?
In the future, if you have magnetic ballasts, the best way to test your bulbs to see if they need to be replaced is hook up a kill-a-watt meter when you first get the bulb, find out how much energy it's using, then periodically after a few months test how many watts it's pulling, you'll see it start to yank more and more power telling you the bulbs are ready to go. Unfortunately my 250w ballasts are electronic, so they're useless with this, my 175w ones however are magnetic.