Is there a guarantee I can stay for 90 days?

With regard to this, please note that all travelers – under certain circumstances – can be denied admission by an official at the border, if they do not meet the VWP requirements. Generally, there is no guarantee for admission, as well as there is no guarantee for the full 90-day period. The officials do have the right to limit your stay. This so-called status will be recorded on your I-94 W.