It'll take at least 1/2 hour from Seal Beach to Disneyland. But, I'd tack on an additional 20-30 minutes for navigating the traffic/parking/taking Disney transportation from parking to the park.
Do yourself a favor: Take 22 east, exit Harbor Blvd. Head North and park in the Toy Story Parking lot. Do NOT go all the way to I-5 North. You will end up being redirected to the TS lot anyway, and it will tack on an extra 30-45 min.
You can be parked at TS lot within 30 min from leaving Seal Beach if you exit Harbor from the 22.
We always take side streets and it takes between 2-30 minutes. I am happy to share our route if you don;t want to drive the freeway. Then plan an additional 30 for parking and walking to the gate.