As
@belledream said, it all depends on how much of HS you want to see. Even before the closure of GMR, my family could cover every single "ride," plus Muppets 4D, two live shows (e.g., Indy, Frozen), a CS meal, the One Man's Dream exhibits and the SW Launch Bay M&Gs, in under 5 hours starting at rope drop. With Toy Story Land, we'd still need far less than a day. However, that means we're skipping all of the other shows (e.g., Disney, Jr., Mermaid, Jedi Training Academy, Beauty & the Beast, Fantasmic! at night) and M&Gs. I would say that if you wanted to do every single show (not that I'd recommend it -- they're not all winners) or meet every character, you would need two days, largely because the showtimes tend to overlap in such a way that it's not possible to do them all in one day. If you're happy just choosing the best-of-the-best and only doing a couple of shows and/or M&Gs along with all of the other "rides" (e.g., Rock 'n Rollercoaster, Tower of Terror, Star Tours, TSMM), one day will be plenty of time, even after Toy Story land opens.
If you want to see how much can realistically be done in HS in a day, you might consider a subscription to touringplans.com, where you can make personalized touring plans for only those attractions you wish to see, customized for your dates/crowd levels. If you're someone who enjoys planning, it's great fun.