Okay - there's a few different TTC buses that go to the airport - I think they all go into Kipling station. From there it would be east on the subway (Kipling's the west end of the line, so you don't have much of a choice about direction anyway
) to St. George station. From there you'd be going south - if you end up going to Hostel Int, it would be all the way to Union, if the one on King W it would be *thinks* St Andrew.
It's a good half hour at least on the bus from the airport to Kipling, depending on traffic and how many stops it makes... From Kipling to St George, 20-30 mins IIRC, and 10 at most from St George to St Andrew or Union. So basically figure an hour to an hour and a half going by TTC.
You can also catch the GO bus and get off at Yorkdale, and subway south from there to St Andrew or Union - depending on when you land it might be a bit faster, but it's also probably triple the price, given you have to pay the GO fare (price depending on your destination) and then the $2.75 for TTC after that.
There's also the "Airport Shuttle" or whatever it's called (the name escapes me at the moment), but that's $11 minimum and I don't recommend it with cheaper ways of getting downtown *g*