There are some decent estimates out here. It really does depend on your definition of max out, and whether you play enough to always have the required resources available as soon as workers are available (workers critical path, fastest) or whether you start upgrades when you get the resources. There is no way to estimate time on the second, since it all depends on your play time.
I will say - most people end up continuing to play this game because of alliances, and often enjoying wars. Being a fully maxed global in war is suboptimal for matching reasons, as you have a lot of war weight without a top notch offense. Ideal is to have your offense well ahead of your defense until it is maxed, and only then does defense start to catch up. There is no reason to aim for being a maxed global - maxed industrial makes some sense (no atomics attacking you) or maxed atomic. But global is only one step away from getting the big upgrades in atomic.