Nothing wrong with spending all day interviewing if you think you have a strong chance of the interview resulting in tens of thousands of dollars in raise/bonus/etc.

Yeah, an actual full day of interviewing is probably more than necessary, but you need to have a strong signal of your ability. I think most technical recruiting at good companies is more about avoiding the possibility of hiring someone who is bad than trying to find the best candidate ever. The business doesn’t want to spend $50k+ on recruiting+paying someone who sucks. So spending a full day on interviewing, especially if you are offering competitive compensation, just makes sense