Sometimes, sounds are lengthened for emphasis. For example, see "とっても versus とても".
What are the rules governing this process?
Are there restrictions on where lengthening can be inserted?
とっても or とてえも or とても～?
すんごい or すご～い or すごいー?
Is it predictable which sound is inserted?
すんごい rather than す～ごい or すっごい?
I've given examples, but I'm hoping to learn if any general rules exist.