As others have alluded to, you want enough to cover the costs of completely outsourcing the on-call and any additional overheads at a minimum.

Support for big customers can often mean dealing with outsourced/contracted employees on the other end who would rather ask for you to 'fix' things than make an effort themselves.

Also, ask yourself one question - Did you build this SaaS to empower yourself, or to be at the beck and call of someone else ?