SBX only takes me about three hours (mega high volume). My first time it took me about four to five hours. I had to get use to the TINV SBX guide and the way some items were counted but its really no different then FA inventory as long as you know where the product is at and as long as its organized.
For tips: make sure two weeks before that the back area and stockroom are being zoned and organized , if its not it will take longer to do inventory and it many lead to wrong numbers. And remember if you have 53 eaches in 14 boxes instead of doing the math you can just type into the PDA 53x14 and the system will do the math for you (saved me lots of time). Since I also starting doing both FA and SBX TINV around 5pm I will usually check out the work centers and tell them to pull out reminding product for the night that way I can start counting stuff in the back and they wont mess up my numbers. And at the end i just have to count off what they didn't use. Don't worry about it to much as long as you don't leave the dept red. The people at HQ auditing the results will usually email the STL if a discrepancy arises which can be fix easily by emailing them the right count.
If I'm not mistaken, the list of "expected items" is some subset of all the dpci's you count. I don't see how you should ever be red if you use the TINV program on Workbench to verify 100% green. IIRC, it will tell you which, if any, of the expected items you forgot to enter a count for.