The Mausoleum is surely one of the best ROI wonders in the game--if you have it, then GAs are a very good use of GPs and a Golden Age strategy is viable, usually allied to another victory strat.
Going that route, my first GA is usually the Taj Mahal, and my second is after getting early Industrialism--the Factories and Power leverage the extra GA hammers, so get 'em up asap. Then you should have 3-4 more GAs to play with, esp if you target the free GP techs.
Without the Mausoleum and a GA strat, I usually only look for one GA during industrialization. Don't forget that a GA gives free civic changes, so if you're building industry or space, then you want to be in Org Rel and Caste, and probably State Property.