Blenheim Palace: how much time?

Don't know what you consider typical. We spent most of the day there, although most of it was outdoors. We probably spent nearly 3 hours on the Palace interior and the related exhibits, although we took our time. I recommend exploring the gardens and grounds if the weather is fine.

I agree you could easily spend all day there and there is much to see both inside & out. I do wonder though what will hold the young teenager's interests though. Are they interested in stately houses &/or history?

I agree that you could easily spend most of a day there (we did) but it really comes down to how interested you are in Churchill family lore and the overall history of the place.
The Orangerie is a terrific place for lunch, and Sir Winston's grave is located literally across the street (just beyond the butterfly house) in the family plot at St. Martin's church.

We did a tour and probably spent a couple hours there doing the portion of the palace open to tours and the grounds. It all depends but unless your teens are into 18th to early 19th century furnishings or gardens I’m not quite sure what would be of interest to 13 year olds. But you didn’t ask for opinions so probably a couple of hours.

We only spent a couple hours, but didn’t walk around the exterior grounds at all. After we visited it we did visit his gravesite at St Martin's Church in nearby Bladon.

This is helpful everyone. Thank you. Admittedly, I'm the Winston fanatic in the family. My kids could make it through an hours but more than that would be a stretch.

Appreciate your thoughts!