There are way too many variables with regard to how you shoot, how often you use the camera's LCD screen for reviewing images or adjusting settings, how much of what you shoot with the combination is in burst mode, etc. to give an accurate prediction.

One thing is fairly certain: Since the grip hold two batteries, you should get twice the number of shots using a grip with two fully charged batteries than with a single fully charged battery in the camera.

I use grips primarily for the set of vertical controls when shooting in portrait mode. The extra battery is just a bonus.

I shoot a lot with the EF 70-200mm f2.8 L IS II on a gripped 7D Mark II. I can shoot 2,000+ images in a sports session over several hours (a full football game including warmups and the halftime show, or 2-3 basketball games, etc.) and have around 50% left in each of the LP-E6 batteries in the grip. A lot of those frames are shot in burst mode so there is no image review for the vast majority of them. I also tend to change settings using the dedicated body buttons and the viewfinder.

If I shoot the same number of images over the course of several days with most of them not shot in burst mode, I may go through a couple of sets of batteries. The 7D Mark II is also pretty well known for sucking the life out of batteries if they're left in the camera in just a few days even when it is turned off (and GPS is disabled).

The Rebel T7i/800D uses a smaller battery, though, and I wouldn't expect you'd get the same performance as I get with a 7D Mark II. The official CIPA rating for the 800D is about 85% of the rating for the 7D Mark II. Canon tends to lowball their CIPA ratings. If you're not shooting half your photos with the built in flash popping you should expect to get up to twice the CIPA rating of 600 frames at 73ºF/23ºC per each fully charged battery.

It's never a bad idea to have plenty of spare batteries. Good, reputable third party batteries like the SterlingTek brand are a very economical way to go. Getting the most recent revisions of the third party batteries may be necessary after new camera models or firmware updates are introduced.