I have cohosts solely for messaging and I would have to say it is virtually impossible to estimate the total number of hours of work a cohost spends on management. The issue is that replying to messages is a 24-hours a day work -- you simply can't limit it to any 8-hour window and tell the cohost not to respond outside his/her work hours. However although it's all-time, most of time during a day the co-host is not doing anything -- I would estimate a total of 3-4 hours worth of work my cohost spends on the 7 listings he's managing per day, but the whole-day standby is also bothersome.
Instead of an hourly rate, we pay our cohost on a commission base -- 20% of the income generated from the bookings. I think this has several benefits. First it gives the cohost incentive to think how to boost bookings and generate more revenue. Also since his/her income is connected to the revenue, he/she has the incentive to treat guests better. An hourly rate might drive some greedy cohost to floor/ceil your daily pricing to maximize/minimize the work he/she has to do, depending on how you evaluate his/her total hours of work. AFAIK most cohosts get paid on a commission base -- the percentage varies by a lot of factors but usually in the range between 10% and 30%.