If you have a work permit (working for your own company) are there any restrictions in the time you could spend abroad?

Would you be allowed to work in Thailand for say 6 months and spend the rest of the year abroad (assuming you have the work permit and meet the income requirements in Thailand)?

Yes, it should be no problem.

For some businesses people spend a lot of time abroad. In that case it is sometimes easier to get a multiple entry non-B, instead of extensions of stay. (Unless you often plan to stay longer than 90 days in a row in Thailand).