Season Unending in Skyrim? How long will the civil war be halted after I trap the dragon in dragonreach?

Not to fret, NOTHING in the game is Time Sensitive. Regardless of how it may seem you can take your time doing anything.

The Civil War is fought only when the Dragonborn is present at a battle.

The only situation where time may affect the game play is during the specific action of a specific part of a mission, which should be obvious.

"Once Season Unending is complete, the Civil War will be on hold until the Main Quest is complete." Source: http://www.uesp.net/wiki/Skyrim:Season_Unending