Understanding C20 value of lead battery for a low power circuit
The C rating often used by RC LiPos is telling you about the ability to deliver the current mentioned. In your case 20A which will empty the Battery in less than 1/20 of an hour so after 3min you're done. The capacity you mentioned is 1Ah so given you draw 1A you'll get 1h of fun (at best). The math isn't really complicated. You mentioned that you expect the average current might be around 90mA so here you go 1Ah/0.09A=11h then your battery is empty. The C rating isn't at all important in your case.