You can leave it for how every long you want, it just depends on how much oil you want to drip. If it isn't hitting the ground yet, then you probably have a little time, but it will only get worse and will need replacement. If it starts leaking bad enough, you can ruin the clutch, though that is an extreme. It is about a 10-11 hour job total, with cost of parts about 12 bucks. Actually while you are in there, you need to do a clutch and have the flywheel turned as well. No point of putting those used parts back in, then having to go change them out again in a few years. The stock boxster clutch is only a few hundred dollars.
Here is a journal of the RMS seal i did a few weeks ago to let you know what is involved.
http://www.turbo911.com/showthread.php?t=220
Oh, and not all porsches leak oil at the RMS seal like you mechanic says. It is an issue with the boxster and 996.
There is a chance that when you replace it, it will leak again. The new TSB with the updated bolts and updated tool for installing the seal helps to prevent that. The tool to seat the RMS has changed, and will now set the seal to a certain depth, which was different that previously.l
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