I'll be driving from Montreal to Manchester in mid to late Sept. MapQuest says this is a 6 hour drive. Any suggestions on a place to stop for lunch ? Is 6 hours realistic ?

We generally agree with times on mapquest and google maps. when I check for Montreal airport to MHT, I'm getting slightly less than 5 hours.

Fancy lunch? slight detour to Simon Pierce in Quechee VT. Aim for as soon as it opens for lunch to get a table by the window.

Quick lunch: café at King Arthur Flour in Norwich VT

Depends on how soon you get started in the morning.

I drove this route several times when I lived in (ugh) Nashua -- it would typically take just a little under 5 hours. (Though that was before the stricter/more time-consuming border controls.)

You have a choice of two routes, both perfectly good: (1)Rte 10 (Autoroute des Cantons de l'Est) to Rte 35, which becomes I-89 at the border all the way to Concord and I-93 to Manchester; or (2)Rte 10 to Rt 55, which becomes I-91, to I-93 to Manchester. The 10->35->89->93 route is probably 5 or 10 miles shorter, but you'll have about 10-15 miles on a slower, 2-lane (undivided) road that goes through some small towns. Both routes have wonderful scenery once you get to Vermont and New Hampshire.

For lunch you've got plenty of choices: On the 10->35->89->93 routes, you can get a quick lunch just off the highway either in Burlington or West Lebabnon NH (just over the border from VT). For nicer lunches, in addition to dfrostnh's suggestions, Hanover NH is a nice town and has multiple choices. On the 10->55->91-93 route, you could stop for a nice lunch in Magog QC, or to visit an especially charming town, North Hatley QC. For a quick lunch midway on that route, your best bets are in St Johnsbury VT or Littleton NH.

I live near Lebanon NH, which is between Montreal and Manchester, and I always take the I-89 route past Burlington to the border to get to Montreal. It takes me a just about 3 hours. Going the other way, I am 90 minutes to Manchester. Therefore Montreal to Manchester via the I-89 route is just under 5 hours, and a very easy drive.
If you get hungry halfway thru Vermont, get off in Montpelier. There is a good Mexican restaurant about 10 minutes from I-89, Julios. It's right down the block from the state capital, so look for the gold dome. If you can make it to the New Hampshire border, there are lots of fast food restaurants at exit 20. For something a bit nicer, get off exit 20 and drive north 10 minutes on rte 12a/10 to Hanover NH - lots of good restaurants like Shushi-Yama and Molly's. Simon Pierce in Quechee Vt is a very good, upscale restaurant about 15-20 minutes from exit 1 in Vermont.