The wheel should spin easily. It may not freewheel for very long, but it should be easy to spin.
The rear may not rotate freely if:
The chain needs lube (even if it doesn't it is a drag on the rear wheel and it will slow it down)
Rear brakes are tight to the disk
Wheel is out of alignment with the chain.
Remove the chain and loosen up the brakes, then see how it spins. Rock the wheel from side to side at the top of the tire and see if there is any play. If there is, it is time for rear wheel bearings. If the bearings feel good (no side to side play), but it freewheels for a long time, it usually means the bearings are dry (grease is gone). You can pull the seals (carefully) and pop the dust covers off of the bearings and pack some grease in them and put is all back together. This is only a temp fix and ultimately you should round up a rear wheel bearing kit. I have been using All Balls bearings from Rocky Mountain MC/ATV since I found them back in 03, and have had no issues.