How much forward depends on the insert, which should determine the wood 'box' into which the insert is placed. This is a ModernAire mounted 72'' from the floor. The counter is 34'' high and 29'' deep. The insert manufacturer specified a maximum height from the cooktop. Higher and the vent will not be as effective.
Here is a picture of the unfinished hood.

You don't mention a brand and model for your vent hood, or the type of cooktop or range you're planning. If you're installing a commercial style range, then a 27" deep hood isn't unusual. If on the other hand, you're installing a cooktop or a non-commercial style range, you probably don't want the hood to extend out beyond the leading edge of your countertops. Range hoods are available in various sizes, so you might want to choose a different model that isn't as deep.

Many people who are planning a decorative outer shroud or mantle type of enclosure choose to install a vent hood liner insert instead of a regular vent hood. A liner insert is designed to be enclosed by a a decorative surround.

Here is a link that might be useful: Vent-A-Hood Liner Inserts

Not only is it normal but it's desirable - the deeper the hood the better the capture area. Also, 30" above the cooktop is what's usually recommended but you can go as high as 36".

I guess I'm worried that somebody cooking at the range would knock their head into the venthood. We do have friends who are over 6' tall. We did get a 52" hood liner/insert from independent hood. It is going over a 36" wide Wolf gas range and a 15" induction hob. I'm just afraid that the hood is going to dominate the kitchen. Ugh. I hope it doesn't look bad.

My Hood sticks out 27", the range sticks out just as much; more if you count the oven door handle. It's no big deal, because it's 72" above the floor, so no head-banging.
Casey

Ours is 24" deep over a cooktop and is mounted 30" off the cooking surface (so that means 66" off the floor). We're a tall family and have not yet had issues with head bumping.

We wanted at least 24" so the front burners would be covered plus a couple of inches more (to capture drifting smoke, etc.) It's also 42" wide and my cooktop is 36" wide.

My brass hood sticks out 24" from the tile (it's hung in front of the tile), 26" including the pot rail. Pretty much what Buehl describes, though the cooktops are farther in from the front edge.

My hood is 27" deep and the lowest portion is 36" above the cooktop. Our kitchen is still a work in process

Ah, you wanted a pic, too. Can do:

Casey

Our range hood is 24" deep over a 24" counter, and 48" wide over a 48" range. It is set 32" above the top of the range; 70-71" from the floor. Our range can be adjusted with the legs from 35-7/8 to 37-5/8 (it's not installed yet).

I definitely think that most hoods extend out two feet.

Did I tell you how much I love GW? As we are nearing the completion of our build, I feel like I have been using it as a crutch to alleviate the worries over all the little things we are stressing about in the home stretch. Thanks for all the great pictures and posts. When our trim guy told me how deep the hood was, I got so worried, especially bc right now the space looks so nice and open, and visualizing a 27" deep by 52" wide solid thing, taking up that much space had me concerned. Ok, I will have faith that it will all look normal.

Thanks!

Mine is going to be 20" deep and 28" off the counter. I was worried till I saw these posts that mine stuck out too far also. My cooktop (electric) counter is 25" deep and only 30" wide. Our ceilings are a standard 8'. Should it be 30"? How do you know if you will bump your head:)

Here's ours and no it does not stick out.

The depth of the hood insert is 19". 21" with the wood. We set it about nine inches higher than specs recommend. It works beautifully and is SO quiet.