Two different success stories (two different ears!)
1. The first time my dog had a hematoma we put had the vet put the drain in. We had the drain removed after 3 weeks (around), per the vet's recommendation. Not a month later it filled back up. So the next time they gave her a steroid injection in her ear. It's been fine ever since though the tip of her ear crinkled up a bit (kind of cute).
2. After this, her other ear had the hematoma. Not wanting to inject her with steriods again so soon, we had the drain put in and, per the vet tech's recommendation, we just left it in indefinitely. It fell out on it's own after maybe 4 or 5 months and has never filled up again.
She didn't even notice the drain (either time) after the first few days so leaving it in for a longer time period seemed to be the favorable treatment.