We were at MK at opening 2 of the days we were there. We didn't arrive until just as the park opened, so we weren't right up front, but we were close. We'd go to Space Mt first. I'd run ahead and get fast passes. There was usually a 30 minute wait by the time I arrived. I'd get fast passes which were for 10ish to 11ish. We'd ride Buzz Lightyear twice (just a few minute wait), go see some other things that didn't have a wait in Adventure Land (Pirates, Jungle Cruise, etc) and we'd come back and ride SM. Later in the day, it always seemed to have a 50 minute or more wait.
Go early and use Fast Pass.
Have fun.