@Anacybele The Aether Raids season begins and ends the same time as normal Arena (used to be one day earlier, but that was recently changed). Only during the reset will you get the aether fully replenished. Upgrading these buildings should be top priority so you can get more gameplay out of the mode per week. You probably know already, but destroying the enemy aether fountain and pot gives you some aether back. This adds up, especially when you consider that the first AR run every day costs no aether.
At the end of the day, AR is ultimately there for us to collect Heroic Grails for us to get more copies of TT and GHB reward units for merges or fodder. I still think it's worth participating in AR even if you have no ambitions to get more merges of these guys, because some do still have decent to good skills that are otherwise locked to 5* exclusives.