How much room on 500GB?

Hi - just got my new Roamio (4-tuner, 500GB, refurb). It says it supports 70 Hrs of HD. I'm moving from my TivoHD that had a 1TB drive so I'm a little concerned. I added up all my Season Passes and all full I would need 80 hours. I know compression is part of the picture - so my question really is - is the 70hr estimate realistic or conservative?

I know I can drop in a 1TB drive trivially, but I'd like to keep the warranty alive for a while (I bought an extended since it's a refurb).

Thanks!
/j