I didn't read the other replies, someone may have already answered this.
No, you are not guaranteed 5 hours a night, peak or not.
If you do not show up at the posted start time....you are late. If you are late, technically, you are not even entitled to your 3.5 hour guarantee (this may vary by local...but that's how it is in mine). Technically, you could show up at 5pm and management would have the right to send you home if fully staffed, regardless of whether lower seniority people were working (who showed up ON TIME).