How long do they take to consume a planet?
Depends on how many are designated to the planet, that opening at the front is Approx 1km by 0.25km and it hoovers in Material at frightening rate but it would tke forever for something that size to go through the volume of a world, and even then it has to take the time to fabricate useful equipment as it goes. The best way would be to DS the planet and then send in the Devastators to convert the asteroid field into something useful.
How many star destroyers would you get from consuming
Well using the following sets of figures : For a planet I'm using the Radius of Earth at the Equator 6378 km. Which will hopefully average out between the Mountains and the sea floor and all the empty space between them. This gives us a Volume of 1347827311392 Cubic km.
For the ISD I'm using a Rectangle 1.6km by 0.6km by 0.6km. This gives a lot of emtpy space but that helps with factoring in all the extra marterials needed to create Hypermatter for the reactor and Neutronium for the hull as I doubt the Devastator can do a 1-1 conversion of ordinary matter to those types of exotics so we're overstating it to take that extra into account. this still only gives a Volume of 480,000,000 Cubic Metres, or a little less than 0.5 cubic km.
So agian the assumption is that the Devastator converts matter to whatever form it needs to for it's building purpose and the extra space used in the ISD volume is to allow for the extra minerals the Devastator would need to mine to make the more Exotic elements in the ISD's construction.
Using these (very) rough figures we get a total of 2,695,654,622,784 ISD's fro one planet. Of course where your going to get the crews for them is another matter.