Most painters bid by the job. Bidding jobs by the hour is a sure way to lose work because nobody wants a bid presented that way. They want to compare one painters bid with the others. Not only that, bidding by the hour is risky like you say....the guy can stretch the job the job out as long as he likes. It's also unprofessional...it's like coming right out and saying to your potential customer, "I have no idea how long this job will take because I don't have that much experience painting so I'm bidding your job by the hour so I can't lose."