Typically it is in Manage My Booking, in the flight details. For most shorthaul services check in opens 2 hours to 90 minutes before departure though it can be 3 hours for larger airports. Longhauls you generally can check in 3 hours before. If this is widely wrong MMB would tell you.

In all cases I can think of, if you check in 90 minutes before departure you'll be fine, the one exception perhaps being Dubai and Punta Cana, around the school holidays and weekend where you'll want a bit longer.

If it's somewhere specific then by all means ask in this forum, someone is likely to know.