The processing time for study permit extension currently is 34 days you can apply maybe a month earlier.
Paper base which will be send to CPC Vegreville is 116 days so maybe a month from that too.
Do not worry because if your study permit expires and your application for extension is already received and will be process your status by then is "implied".
You will be in implied status for 90 days or until the decision has been made for your extension.
Sometimes you will be rejected for applying too early.