It's up to the maintainer to offer up an SLA and price it accordingly. Try to figure out the SLA you get for your windows or office license: Many companies would already be happy with "our payment places this otherwise uninteresting issue that we desperately need fixed at the top of the queue", even if no guarantee for a time frame was attached. If a company expects 24x7 with short response times, price it accordingly. If you find customers willing to pay for this, great, you just validated a market :)