You want to open up enough room so that you can easily work in the area. If your hose is already pretty short (meaning there's little or no slack) then you're all done!

Angie's How To Shorten Your Dryer Vent Hose
If the vent hose that connects your dryer to the wall duct is too long, your dryer has to work harder to pump steam away, which costs you extra money. By shortening the hose as much as possible, you'll reduce the energy required to do laundry and save money on your utilities. As an added bonus, your clothes will dry faster and your dryer will last longer!
3 pros!
Take a quick look for kinks, holes or other damage (if you find any, it may be worth replacing your hose entirely). Then, carefully detach the vent hose.
Optional: This is a great time to clean your vent and duct by vacuuming the back of the dryer, the dryer duct and the wall duct.
Use your measuring tape to measure the distance between the place your dryer sits (remember, this is different from where it's sitting while pulled away from the wall). Then add six inches to make sure there's enough room to make the connection on both sides.
Measure the distance you came up with in step #3 on your vent hose and mark it with a strip of painter's tape.
Tip: Wrap the strip all the way around your hose and draw a line through the middle so that it's easier to cut a straight edge.
Put on your gloves to protect your hands from sharp metal. Then trim your vent hose with your tin snips. Carefully remove the painter's tape when you're done cutting.
Carefully reattach your vent hose to the dryer. Then attach it to the wall, scooting your dryer back towards the wall as necessary.