How long a tape stays in drive after write depends on a parameter
"mountretention" which is defined in "deviceclass". I don't know
how a single drive is handled, but I would look here first.

Using adsm with just one single drive - hmm. If you are using
a disk pool you can fill him at night and empty him over the
day with upd stgp <diskpool> hi=0 lo=1 (or so). So you can
control when migration happens. (But your disk pool must have
enough capacity).

Regards
Roger