if it's not leaking enough to leave a spot on your driveway then jam that rubber plug back in and check your oil regularly.
or if you have a lot of time and/or money on your hands then you could pull the transmission off and fix your leak.
mine has had a leak for about 4 years and it will probably still have that same leak 4 years from now. a leaky rear main seal is quite common and there aren't too many vehicles out on the road that don't have that same leak.