would welcome any suggestions on how many days each of the inscribed sites require to visit
The length of time you give each depends of course on the total amount of time you have - I wouldn't disagree with Els's "proportions". Each of Samarkand, Bukhara and Khiva can each be "done" in a day - though Samarkand's full list of sites of interest covers quite a large area. So, add available time to Bukhara and Samarkand before Khiva. Shakhrisyabz is certainly a part day "visit" destination - again, depending on your transport arrangements, there is no absolute need to return to Samarkand if you are going on to Bukhara as you are already part way there! You would need to check on latest transport details.
A question of course is what else could you do in the area. I don't remember Tashkent having anything of note - other than an extra "tick" on our "Metros of the World" list! We certainly value the memory of our trip up to the "Aral Sea" from Khiva - a unique experience to see the "beached" ships sitting in the middle of today's "desert" near Munak. We had a car with driver and drove from Khiva to Munak and back to Nukus (though could have got all the way back in a day). We stayed over in Nukus, however, as we wanted to see the Savitsky Museum there with its collection of avant garde art "saved" surreptitiously from Stalin etc (see the Web) - but I don't know if you are interested in such things.
As for a trip across to Kazakhstan - we were not able to do this back in 2004 (we were concentrating on the Uzbek, Kyrgyz and Turkmen "Stans") but I understand that, until 2017 at least, Kazakhstan is offering Visa free entry to European nationals so, if you have sorted out double entry visas back into Uzbekistan, it is certainly possible. But what do you do when you have crossed over - other than pick up another "country"? We visited Almaty during Soviet times - it is nicely situated but not really accessible on a short term crossing from e.g Tashkent. As well as the inscribed site of Turkistan (c320kms away) there are the closer T List ruins of Otrar "only" around 230kms north of Tashkent. but .........? It would need research to see if it could be arranged from Tashkent and how long it would take.