Had brain mri today with contrast. how soon would i know if i had renal failure or delayed reaction? what would i look for?

Improbable: Both renal failure and delayed reaction to MRI are rare. I have ordered perhaps 10,000 MRI's and never seen either. Is there a reason you have these fears? Effects of contrast dye show up within hours although rare renal failure might show on tests in a few days. Talk to your doctor. You can have renal tests if that seems really necessary.

Contrast reaction: If you have normal renal function before the MRI with contrast your risk is very small. Patients with severe renal dysfunction can develop renal failure, skin thickening in a few days up to six months after the contrast administration.