I agree on making the Signal Fire spawn 2 fixed batallions every 4 minutes (and raising the number to 4 when you purchase Assistance in the time of need)^^
However, since units now spawn in the location of the Signal Fire, granting them upgrades would be slightly unbalanced if you can build a Signal Fire close to the enemy base, and after 4 minutes you can get 2 (or 4) batallions fully upgraded. So, here's my idea:
The standard waiting time is 4 minutes. For each upgrade purchased by the player, the waiting time is increased by 2 minutes, but the troops will spawn already upgraded (the more upgrades you purchase, the longer they take to properly equip and reach the battlefield)
So, if you purchase all the four Gondor unit upgrades (not counting Knights Shield), the units will take 12 minutes to arrive (4 by default, plus 2x4 unit upgrades), but they'll spawn fully upgraded in the location of the Signal Fire