I have mono and concerned with my spleen rupturing how long until i can safely return to doing physical activity like going to the gym or having sex?

You have misunderstood something you read or found online. You need to discuss this with the doctor who diagnosed your mono and is treating you. The spleen can be enlarged in mono, but there is risk of rupture only if there is unusually great enlargement. If not, exercise etc is safe. And no matter how big your spleen is, there is non risk of splenic rupture from having intercourse.