I suppose it depends on the indiviual dogs. At the centre I work at, we start at 8am and go home at 5pm, so the dogs are left alone for 15 hours. A lot of them are fine, some even wait that long for the toilet.
I'd guess that they'd keep each other company, and as long as they have toys and food to occupy them, they should be fine.
Have you ever had trouble with them being destructive?
Edit: unless you have time to slowly leave them for longer and longer periods, the only way you'll know is try it. The worst they could do is wreck your home