Once you have sent your troops, a countdown on the troop march will appear on your screen .

When your Messages box starts flashing you can check the Battle Report (via Messages – View Reports) to see if you won.

If you defeated the wilderness, your troops should be listed as encamped
(unless you have already conquered the number permitted by your castle size).

You do not need to leave them there any length of time:
If you go to your Rally Point, click on Troop Movement, and “Recall” the troops, your will still control the wilderness.

If you go to the wilderness directly and “Abandon” the province, you will not control it any more.