I agree with RSM. At least 4 hours actually *at* the Harry Potter Studios tour (we spent a little over 5 hours last month). There's a LOT to see & take in, and if you're a fan, you really don't want to rush through it. It takes about 30 - 45 minutes give or take to get there from London Euston Station (maybe 15 minutes on the train, wait for a shuttle, 15 minutes on the shuttle). Plus whatever travel time to get to London Euston Station. And allow time to get Butterbeer ice cream at the half-way mark, and to go crazy at the gift shop. Really, really special for any Harry Potter fan! It can take most of the day. We left around 9-ish, hung around & had breakfast while waiting for our 11am entrance time, then spent a bit over 5 hours there, leaving a bit after 4:30pm. By the time we got back to London, it was dinner time, and then we did the Ceremony of the Keys that night. A nice, full day.
You can easily do the Sherlock Holmes related stuff on a second day. We did it ourselves, no tour, and we still managed to do it in less than a day.
Sayhello