tomorrow, if at all possible.
in all seriousness, i'd say when you hit 1Mbit/s average it's probably wise to move to a dedicated server. of course, it ultimately depends on how loaded your shared server is - if the other sites on it are holding yours back, clearly, move ASAP. but if they're all non-script sites just pushin content, it shouldn't hurt to stay there.
basically, if you're comfortable with your site's performance now, there's no need to move. if you're worried others on the server will hold you back, move. and if you get a good deal on dedicated hosting (sometimes you'll get a lower bw cost, even if you're payin to rent a server), take it.
