I can imagine this will be similar to many strategy games, where the less experienced you are the less time a match will take because of the mistakes you will likely make. As you get better you will make less and the matches will take more time, then once you're experienced enough and have enough tactics, you will start winning quicker. I'm thinking a match for a beginner will be around 10-15 minutes, intermediate will be maybe 25-40 (maybe stretching it), and pro will take 10-20. It depends on your opponent as well, so I can imagine a match could take quite a long time.
As you gather more resources you can summon more monsters, and stronger ones, so once you have around 7 of multiple resources, you'll be able to summon almost any scroll and the game will be sort of "sudden death." Any mistake you make at this point will probably mean a loss.