As noted previously, a lot depends on what you mean by "comfortable."
My wife & I travel about 4-6 months a year, mostly in 1-2 month chunks. Since semi-retiring about 10 years, ago, that's included a half dozen 1-2 month stays in Paris, two or three 2-4 week stays in Nice, four or five 3-5 week stays in Switzerland & Austria, two or three 3-5 week trips to Spain & Portugal, one 5 week stay in Florence, two or three 1-2 month stays in Argentina, Chile and Brazil, five or six 2-4 week cruises (Iceland/Greenland/Norway, Argentina/Chile, Brazil/Caribbean, Buenos Aires to Barcelona, Miami to Barcelona, etc), and three or four 1 month stays in Salt Lake City. We split the rest of our time between our residence in Florida (where siblings & MIL live) and our previous home in DC (where the kids & grand kids live).
In Paris and Nice and Vienna and Florence, we've rented 1-2 bedroom apartments (depending on whether we expect houseguests) for about $3,000-5000/month. These have been comfortable apartments i(with W/D, AC in Florence, 1 to 1.5 bath, etc,), but "luxury" would have been a couple of thousand a month more. We've lived in the 18th arrondissement in Paris (Montmartre), where my wife has been taking art classes), in the old city and just off the beach near the Hotel Negresco in Nice, in Landstrasse (3rd district) just outside the old city in Vienna, and near the soccer stadium in Florence (a bit outside the center, but walking distance to DW's art classes). And, in general, we found meals and food and entertainment to be less expensive in Europe than in the US.
We're hikers and we've stayed a week or more at a time in numerous places in the Alps and haven't found it particularly outrageous. For example we stayed twice in a beautiful family owned hotel 3* hotel iin the Italian Dolomites with half board for $1400/week, in Kitzbuhl, Austria for about the same price but just with breakfast, and 3 or 4 times at Sunstar hotels in Switzerland (a local 4* hotel chain), hiking from hotel to hotel in the Graubundin (Klosters, Davos, etc.) and the Bernese Oberland (Grindelwald, Wengin, etc.) including luggage transfers and gourmet demi-board for about $3,000 per week. We've also used Sunstar 2 or 3 times for ski vacations in the winter for about the same price.
We've sometimes used wholesalers (usually Natural Adventures) for 7-10 day hiking and/or biking packages during the course of longer trips. These have included the Danube bike path (Passau to Vienna), walking the Camino de Santiago and hiking in the Costa Brava in Spain and biking the Algarve in Portugal. These are typically $1500-2000 a week for the two of us, including luggage transfers, 2 or 3 star hotel, breakfast, bikes, etc. (a few hundred dollars more when we've rented electric bikes).
South America and Asia (where we lived for a number of years) are probably 1/2 to 2/3 the price (and, in Asia at least, you can get real luxury for about the same price as "comfortable" in Europe.). Luxury cruises can be a bit more, but with the flexibility of retirement we were able to cruise for 22 days on Viking from Buenos Aires to Barcelona for about $10,000; for about $25,000 for 40 days on Viking from NY to Rejkavik via Greenland then continuing on Hurtigrturn (in a luxury suite) from Bergen to Svalbard and back with 3 day land stays in Reykavik, Bergen and Oslo; or about $30,000 for 30 days on Silverseas (with butler and suite) from Rio, up the Amazon and through the Caribbean to Ft. Lauderdale.
So, if you just want to experience a "comfortable" month in Paris or Vienna or Barcelona, you can probably do it for $5-6000/month all in. One advantage of longer stays is access to local deals: for $80-90 you can buy a one-month Paris transit pass good for unlimited travel within 50 km of Paris (including Chantilly, Versaille, Fountainbleu, Le vaux de comté, Malmaison, etc.) and for $160 you can buy a dual membership giving unlimited access to the Louvre (without lines) through a special members entrance!. Also check what kind of access your local cultural memberships may get you. For example, our membership in the Barnes (Philadelphia) gives us reciprocal access to the D'Orsay and L'Orangerie in Paris and the Art Institure in Chicago, among many others!
If you want to stay in comfortable hotels in tourist areas and/or book hiking/biking packages through wholesales, the cost may be more like $10,000 per month.
And, if you want to spend your time on luxury cruises that'll be $15-30,000 per month (though still comfortable lines like Celebrity or Princess or. Royal Caribbean can be much less expensive...)
Slower travel definitely has a lot to recommend it. We prefer to combine a mixture of 1 week to 1 month experiences within a 1-2 month trip. When we've stayed longer, we've generally been involved in ongoing activities like my wife's art classes and my language courses in Paris....
Enjoy!
