Well, it's quite hard to be accurate as depending on the compression ratio used in different programs, one can't say that 1 hour of HD will take up xxGb of space, so I can offer the following wisdom :
Installing a 750Gb drive will leave you with 610Gb storage (750Gb minus AnyTime reserved space).
The last drive I installed was a 500Gb job and that was tested to reveal 200 hours of SD or a tad under 75 hours of HD broadcasts.