You can use the free plan as long as you can. Still, you can only create and edit 30 designs. The free limit is designed so that everyone can try doographics before buying.
Updating from the free plan will allow you to create more than 30 designs and increase storage area and many provide many several features.
Yes, you can cancel at any time. you can use the plan until it ends.
Yes, if you don't cancel the auto-renewal, you will be charged on a monthly or yearly basis depending on your subscription type. If you only need it for one month you can make the purchase then cancel the auto-renewal from your billing profile.