Yeah it is SUCH a grind hahaha! I retired after I got level 1000 in Athena, I'm just casually levelling up my reapers whenever I feel like it but omg it does go slowly lol. Best time to hourglass is during community weekend when reputation is boosted!
I think it depends how long you're playing each time, how many flags you're selling (flags are GREAT for reputation!!) but maybe 1-2 months of consistent playing? It's always better to play during gold & glory when reputation is boosted!