When I picked up my bike at the port, the official asked me how long it would remain in Thailand. I told him I planned to take it out within 3 months. So he gave me a 3-mo. permit, and he told me I could renew it for 30 days at a time up to a max. of 1 year.
Note that any time you leave the country with the vehicle, the case gets closed, and a new permit is issued upon re-entry. Hence, I assume you can keep your bike on a succession of temporary import permits indefinitely, but this loop hole may not remain open if it leads to too much publicity or abuse. Already, some customs officers seem to think it's a form of tax evasion.