Will alliances have a time limit or last for as long as the leaders want, and will there be a max number of alliances allowed per guild?

Alliances don’t work exactly this way. The “alliance” term, in its current concept, refers to the side you join when entering the combat zone around an outpost. An “alliance” (a side) can include any homin, without number limitation, and your part in the alliance ends as soon as you leave the combat zone. Beyond this point, an “alliance” simply exists if you decide to make an attack, with whoever you want. It has not a defined group from a gameplay standpoint.

How is such an “alliance” created? Xavier already described this system in the Outposts Q&A. Basically, when you’re entering the outpost battle zone while a battle round is occurring, a window will pop up asking you to choose a side (defenders/attackers) or stay neutral.