I don't think you can increase the amount of time the tool tip stays up.
We did a form for a client one time a year or so ago that implemented a tool tip in the taskpane (DHTML) and my developers had a lot more control over the tool tip there than in the default InfoPath tool tips. You might want to think about something like this as it would give you more flexibility I think.
How much content do you want in the tooltip?