Nope, you can't.  But you can of course make your tooltip with DIV.
The topic was discussed before in this list and somebody had posted a 
URL.

> -----Message d'origine-----
> De : lilia lopez [mailto:lilialopez47@h...]
> Envoy=C3=A9 : mardi 28 janvier 2003 22:49
> =C3=80 : javascript
> Objet : [javascript] how can I increase the displaying time
> for tooltip
> in HTML?
>
>
> Hi,
> I'm using the tooltip, the default time for it is 10 seconds,
> how can I increase this time?