If a blister/ulcer on floor of my mouth doesn't heal after few weeks, does that mean chances of oral cancer are very high?

No. Don't assume the worst right away. You're right that most oral sores seem to heal within a few weeks, but that's just a general guideline. Make sure you're not doing anything to aggravate healing like frequent rinsing with an alcohol based rinse. I would have the area checked out though to see if it warrants further investigation.