You say this as if most (or any) candidates could do this.
If you currently have a job, then you almost certainly won't have time for this, unless you're single with no hobbies. If you do have time, you may not be allowed to moonlight under your current employment contract.
And if you don't have a job, you _still_ may not have the time or desire to do this!
I'm currently jobless (by choice - I wanted a break) and I started interviewing a few weeks ago. It's exhausting! Even if I could squeeze in 30 hours of work over a couple of weeks, I wouldn't want to. I had 11.5 hours of interviews this past week, and now you want me to spend another 10-15 hours pair programming with you? Absolutely not.
If you spread those hours out over many weeks (6 weeks at 5 hours per week) the candidate will be done with the process everywhere else much sooner and they'll just accept an offer before this process finishes.
If you want a trial period, make an offer and have a 3-month probation period during which you will give the new hire regular feedback (at least once a month but more often is better). Doesn't every company do this already, at least implicitly?
