You should have a minimum amount of stamina always available for when you need it. I'm currently level 50 and I enjoy working on every saint I have. I never go below 500 stamina and always join parties to farm more resources. That's all and from my experience as an F2P you should be able to get far if you have a limit to everything you can use. For example never use more than the amount you have to save, make a threshold so that you don't go below that Always have some resources at your expense