The Medal expansions run out when you buy the last one (that costs 1,100,000 Medals). No more after that.
At that point Medals become relatively worthless except for GvG, and buying attempts in GE
You can buy the expansions bought with Medals as fast as you can collect the Medals, in any Age.
The best mechanism for getting Medals seems to be to take the top 1.9 Reward positions in high Era Great Buildings. The rewards include a lot of Medals. 30,000, 50,000 or more a pop.