Well, it'll be quickER and easiER than if you were starting from scratch. I had internet service first, and later added TV. My internet install took about 4 hours, and my TV install about 2 1/2. Then again, my TV installer had to download the s/w to the STBs twice - the first time it didn't take on one of my STBs because he didn't have it plugged in all the way. And considering that it takes a good 20 minutes just to download the s/w...
Also, if you're an older internet install, it's possible that they have you hooked up with ethernet for your internet connection. If so, then they need to run one more cable - from your computer router to your TV splitter, to handle guide data, VOD, and other IP-fed stuff. If you're set up with MoCA for internet, it's easier - which is probably why Verizon has moved totally to MoCA for all internet installs. In that case, the cable is already running to your router, and no additional cable to that router is necessary.
Granted, it'll be alot faster than your first install - but it'll still take some time.