Originally Posted by GUWonder

You'll easily have at least 7-10 hours to see things in central Stockholm. After landing of my US-ARN flights and getting off the plane at Arlanda without checked baggage to claim at ARN, I'm in central Stockholm in 26-60 minutes. If you budget 60-75 minutes for travel time each way and 30 minutes to clear security and get to the ARN-PRG gate, it should work fine assuming you've checked in for the ARN-PRG flight before leaving ARN for central Stockholm. Pad 15-30 minutes more in each direction and it should be way more than enough. Buying the roundtrip transport for ARN-city center at ARN would be a time saver.

I think you would be able to get a good 8 hours of sightseeing in central Stockholm without issue.