Excel 2007 counting across rows to the next blank cell

Hi There,

I'm looking at writing a formula to count the number of days worked since an employees last day off (results to appear in E7)

From how my spreadsheet is designed, I'm needing to count the number of start/finish cells in Row 7 (F7:G7,J7:K7, and so on) until I get to a blank start/finish cell (and then I'm assuming I'll need to divide by 2 at the end of the formula to get my total number of days) I then need to start counting again once the spreadsheet is update till the next blank cell.

Is this possible? I'm only recently delving into the more complicate formula so any advice/suggestions would be appreciated