How long can I leave mead in the primary?
Depends on many things (too many to be listed).......
Some like to leave it until it gets to a certain point in the ferment, some like to move a batch to a secondary to finish a ferment if it's got fruit in it and they want to add more (to bring up the fruit colour/flavour/aroma to a higher level).
Some just move a batch, to free up a fermenter i.e. bucket start/primary, Carboy/Jug/Demi-john to secondary/finish/rack/clear etc etc.
If you're just making traditionals, then it might not be necessary to change fermenter until the batch has finished and needs racking - which is what beerme70 seems to be alluding to.
Of course, it might also depend on whether you need (or want) to use finings etc or not. There's a lot of mead makers, me included, who prefer, if possible to let any hazes, sediment, etc etc, to drop out naturally, over time. Though this might not be possible if you're short of brewing space etc etc, so you might need to rack it off a sediment and then hit it with finings and so on.....
beerme70 is spot on about ageing though and the relevance of patience. I'd say that 6 months is the absolute minimum i.e. the first stage at which you'd check to see if the batch is to your liking or not.