That can vary greatly. How so? Well, consider travell time - the time you spend travelling between jobs - these are unbillable, but the employee is still earning.
If, in an eight hour work day, together you serviced 12 clients (should be easy with two), and you 'lost' just five minutes between clients, thats 12.5% of the day. Not only that, but you only got 7 billable hours to pay him eight!
If you trust your casual to work unsupervised, it is better to leave him at a larger job and go do smaller ones yourself.
If your market can bear $40/hour, likely your employee will cost you 75% of that.
Check some other comments on this topic here
http://www.indmowing.com.au/forum/showthread.php?t=4355