Northern Italy.

It depends on the details. First blush is a month for all of those places in their entirety? Easy travel by train/bus/ferry, unless the Tuscany portion requires a car (villages/countryside). Flesh out specifics for each part, then see if you can do it all in the time you have.

Pretty open ended question but I would say around 3 weeks for this kind of schedule.
Milan, Venice and a town on Lake Como are specific locations that I would say require a minimum of 3 nights each and these can be visited by train.
The "Dolomites" and rural "Tuscany" are large regions that generally require a car to fully explore and require more time because of the size and moving around. If you figure a minimum of 4 nights each that's 17 or so nights with some travel night concerns so I think three weeks would be comfortable and unrushed. All of these locations - except Milan - encourage relaxation and exploration so to enjoy them make sure you have adequate time.

You could fly into Venice and out of Milan - Venice > Dolomites > Tuscany > Lake Como > Milan and fly out. This would put the car parts together - rent in Venice or nearby and drop in Tuscany (maybe Florence) - and get back on the train.

You might check out Lake Garda which is different beautiful lake but is in the Venice area near the Dolomites so it's clustering your interests together. Or chose Lake Maggiore which is close to Milan's airport Malpensa (MXP) and you could do Milan > Lake Maggiore > airport and end on a relaxed note instead of big city Milan if you want.

My $.02, have a great trip,
=Tod

For a reasonably accurate answer to this question, we really need to know what you want to see and do in those places. Milan has a lot of art museums. If they are of no interest, you probably don't need more than a few nights there. Do you plan to hike in the Dolomites? That calls for extra time. Tuscany, as already mentioned, is a large area, and it has a lot of attractive destinations. I'd spend some time with a guidebook, building lists of must-see and maybe-see towns. Without that, it's difficult to give any kind of estimate. The Venice Biennale will be running during the time of your trip. For art lovers, that adds at least two days to the time otherwise needed to see Venice's highlights.

I would take a Rick Steves Village Italy tour (13 days, starts near Venice in Paudua ) and do Venice and Dolomites on front end. I would end in Milan from Lake Orta. Happy traveling