Ideally vent ducts are as short as possible but few situations are ideal. Plenty of people have duct runs that are as long as yours or longer - think about the run from a kitchen in a two story house up to the roof, for example. When installing your duct, consider oversizing it based on the hood you might have in the future. Your microhood will have no problem with a duct bigger than it needs and the added cost will be minimal.

To minimize grease particulate precipitation on the duct walls, the air velocity should be in the recommended range of 1000 to 2000 feet per minute, so the duct size should be commensurate with the fan capability. (Divide fan cubic feet per minute by the area of the duct in square feet. Use the estimated air flow for the duct and hood losses, and not the fan's static pressure rating.)

While commercial ducting has to be welded together to keep grease fires out of the building structure, residential ducting is usually slip-jointed. So occasional inspection of the duct interior would seem to be a good idea.

I'm not sure what defines a microhood, but if you want to keep down odor and grease laden air, then the hood has to achieve both capture and containment. Capture is mainly determined by the hood entrance area exceeding the area of the burners, while containment is mostly determined by hood volume and exhaust flow.

On the other hand, if the link below is apropos, then you may not need to vent the hood at all.

kas

Here is a link that might be useful: amusing thread