I've had them in for months and have not had a problem. Does depend on the brand though. Kjw mags are quite reliable and havent had the seals fails me after all these years. Kwc though... A few have failed a few days in.
back on topic I personally would not leave bulbs in mags for any length of time, but with airsoft it could wreck your seals in a day or stay gas tight for years. for the cost of a bulb is it worth finding out?