Here's a chart I found for 350 degrees that seems right based on my experiences:
You said you were going at 300, so I'd add an hour to their suggestion of 3.5-4 hours, which is what my time is between for a 350 degree cook.
You could bump it up to 325 too.
Either way, I'd guess at least 4 hours for a 20 lber. Probably more like 5 at 300. Just check it at 2 hours and see if it's about 1/2 way, then adjust temps as necessary.