How often do you get Botox between your eyebrows?

Hello! Lately, perhaps because of the heat, I've been frowning a lot, so I'm considering getting Botox injections between my eyebrows. How long does the effect usually last when I get Botox for the first time? I'm not sure how often I should get treatments to maintain a natural look. I know it probably varies from person to person, though. If I get Botox several times and consistently, will the effect last longer later on? Or do I need to continue getting treatments at a similar interval? Since I don't know much about Botox procedures, I'm a bit worried about developing resistance. I think it would be best to maintain a schedule and get treatments somewhat consistently.