Is eight minutes exercise still enough if you have been doing it a while or do you need to increase the amount of time?

That is up to how you feel. If the eight minutes is effective for you and makes you happy then stick with that small goal to start. If you feel ready to challenge yourself and are unsatisfied with 8 minutes, then I would add a little more time. Good luck!

It's completely up to you. Do what you feel like. If you want to have a 30 minutes intense workout, go for it. If you don't, then don't do it.Most importantly, listen to your body.