The OldGrindBot implementation will check the Instance Manager gui for a certain number of instances to help catch any issues with plugins, or other bugs where too many instances could be created. There is no "instance limit per hour" feature/limitation itself though. If you're running into a TooManyInstances error, then you can change the default, which is 10, to something higher. You just need to execute some code:
Code:
Loki.Bot.Logic.Bots.OldGrindBot.TravelToGrindZoneTask.MaxInstances = 20; // or w/e value you want
You can do it in Start in a plugin or routine and you should be good to go.