It usually takes between 4 and 6 weeks for the cycle to complete, from start to finish, but it varies for everybody.
Your ammonia and nitrite will rise, and then fall back to 0. Monitor them all the way. When ammonia and nitrite are at 0, and steady for a week or so, you're good to go. Nitrates will also have risen to a certain degree - they're less deadly, but should still be kept in check with regular water changes after the cycle.
Good decision re. the tang, it's the right one
With ich, the tank needs to be left without a fish host for 6 weeks to be sure the parasite has gone. It can't live in inverts so they're fine to be left in there. But, you still shouldn't have any inverts in your tank during the cycle. Do you have inverts?
If you've got inverts, to be honest I'd take those back too, with the tang, let your cycle finish, (and leave the tank empty for 6 weeks), and start again mate
Remember, before you do start again, use the time while the cycle is finishing (and preferably longer) to research all you possibly can.