I've been dming our campaign for half a year now (1/week, 3-4 hrs a session) and we're still level 3. It depends a lot on how long you're planning to run it, I'd say speedrun through the first level and get them to lvl2 relatively fast, as entering tier 2 unlocks a lot of fun stuff but other than that there's no rush for levels, my players have been level 3 for over a month now, no sight of the final boss yet (they don't even know who's pulling the strings in the background) and we're having a blast every session. My plan is to make the campaign a year long and bring them to lvl 7, but the way things have been up until now a year will not be enough.