There are lots of different estimates for this, but no-one knows for sure because our usage is changing all the time, and there are still deposits of oil and coal to be found (as well as gas! don’t forget gas).
It seems like there is enough for a while yet, and progress towards clean and renewable energy is going extremely well! So hopefully we’ll eventually be able to make the jump.
As Mike said, no one really knows because we’ve probably not found all the oil coal and gas yet. Hopefully we will be able to use renewable energy for all our power needs before any of the fossil fuels (like oil and coal) run out.