We have always been able to get good Diamond Parking places even as late as early in the afternoon. But then again we typically go during the value season. We drive to the parks even when we stay on site, because we HATE the buses.
On average you have to wait 10 minutes for a bus to come, and that is during the day. At park closing it can sometimes take up to 2 hours to get a bus back to your resort. Then there was the INCIDENT. I'll get to that in a minute. We have found it MUCH faster to just drive ourselves.
Ok THE INCIDENT:
A few years back, we were leaving one of the parks at closing (I think it was Epcot). At the time our kids were 1 and 5. As it turned out we had to wait about 45 mins to an hour to get on a bus. When we were finally able to get on a bus, there was only one seat available. Both my kids were exhausted and the one year old was already asleep. So I let my wife have the seat, and she held our 5 year old who promptly fell asleep before we even got out of the bus area. That left me holding my sleeping 1 year old, STANDING. There were several well bodied men and older boys that were siting all around us and NO ONE offered a seat. I understand the legal implications, if a WDW bus driver asked someone to stand so that a child could sit, and that person got injured as a result. But I honestly think that the PR nightmare as a result of a child getting injured as a result of HAVING to stand would be much worse. At the time, and to some degree I still now, I don't understand why the bus driver couldn't have made an announcement as follows, "There are several children and/or parent holding children that are currently having to stand. If anyone that is seated feels comfortable standing, please consider giving up your seat to one of them." Worded in the right was this would have absolved Disney of virtually any responsibility. On the other hand I was complete taken back by the number of people (mainly men and teenage boys) that could have easily stood, yet didn't even seem to consider it. I suppose I could have waited another 15 minutes for another bus, but we were tired, our kids were tired and weren't exactly thinking clearly.
Since then we have only ridden WDW buses when absolutely necessary.