Any time you do it, it it is better than not doing it a all, but for the best results it it recommended 20 minutes and don't exceed the 20 minutes since the bacteria gets restored in your mouth again.
I was afraid to try it out too because I thought there is no way I do 20 minutes, but it isn't like gargling but just simply swirling in your mouth in the morning (the best time) while you are showering and getting ready for the day, and it is actually really easy and the time goes so fast, that you will not even know how fast it went.
As for the essential oils, I don't think that there are better alternatives, but if you can't get them then as long as you just eat healthy foods and just take care of your health as much as you can.
Before I got into the oils, I would also use natural path doctors instead of normal family doctor since natural path doctors prescribe natural medicine too and they can really help you out with whatever you are dealing with, but having the oils at home and using them daily, helps your body, by making the immune system strong and making it healthy, so that it doesn't get sick and makes your body heal itself.