To orbit and back multiple times. In and out of that area many times. Sat and waited until booted for inactivity several times. If the spawn rate is that low, it’s too low.
I had an early nap so was up at 3 AM playing, as soon as I dropped in, literally, as soon as I dropped in the Servitor was right in front of me. Running around, doing Devrim's bounties, it spawned 3 more times. Just hang out, kill things, do other things, it will be right there for you.