Since you're in business presumably you'll be among the first to immigration, so most likely will be clear in less than 15 minutes. If you somehow end up at the back of the queue though, they are not speedy.

What are the conditions of the domestic ticket? Can you change it if you miss your flight? If so I'd probably book about 2 hours from scheduled arrival and hope for the best. If it's absolutely non-changable or domestic flights are typically sold out then I'd leave more of a pad just in case, maybe up to 4 hours. Unfortunately the airport is a ways out of town and there is nothing to do around it.