Team Valor member here, we managed to set up a level 7 gym, with pokemon around 1000 cp and a Lapras with 2000. We held it for two days before Team Mystic finally took over. It's a constant fight between Valor and Mystic out here, with Instinct not having much influence. I think the only way to hold a gym for any amount of time is to meet up with other players and organize, otherwise your gym is gonna get taken really quickly. Fun game, i'm pretty excited to see what's in store for the future updates, especially if it's related to the three teams.