Definitely stay a couple of days. You can spend a day just looking round Diocletian's Palace.... and the bars at night are great.

That part of europe is beautifull stay for couple of days...

take a couple of days, walk around the old town and enjoy the new funky waterfront, and try to find some 'hole in the wall' bars to make friends with the locals, who are all friendly.

that depends on what you are interested in - you can stay an afternoon and get to know the details about the Diocletian's Palace easily. However, that won't be enough time to enjoy Split (restaurants, bars, shops, the mood). But that's up to you. Suggestion - take a trip to Trogir, around 35km from Split, Unesco heritage world site, worth a visit (morning or afternoon, slowly to enjoy the details and the narrow streets)

A few hours in Split is enough to see the main sights. But if you wish to explore the Marjan park you most deff need more time.

I would spend the night. Hvar is pretty cool. It is beautiful.

adam is right, the cultural aspects are not satisfied in only 3 hours. also the nightclubs are amazing!

I've spent there only half a day - appr 5 hours and it was quite good, most of the time walking in Diocletians' palace

If you are going to be there in the summer stay the night. The streets are busy and fun to walk at night.

Unfortunately I was there late in the evening and there was only one bar open (it was a Tuesday night). The shops and waterfront looked lovely and I would have like to stroll about during the day. I think that 4-5 hours would probably be just fine for there. I will recommend, however, the hotel President as it is a very nice and unique place to stay. The lobby isn't too spectacular but it has some great rooms, many of them with jacuzzi tubs.

Thanks a lot everyone. The tips are quite helpful.