When you're traveling slowly, it's tempting to think you have no limits on your trip. But some limits become obvious early on. For example, Americans can't stay in the European Union more than 90 days on a simple tourist visa. Staying longer involves much more preparation, paperwork, and bureaucracy.
In general, the longer you stay in one place, the less expensive your trip is on a per-day basis - your flight is usually the biggest cost, but that gets spread across the entire time. Renting an apartment for over a week usually saves something, and a month almost always saves more. The same is true of renting a car, or buying a monthly rail or bus pass. So, staying for at least a month can save on daily expenses.
However, we found some practical limits that aren't very obvious. The one that is foremost in my mind is car insurance (yep, read all about it here). If you're counting on your credit card to supply your collision insurance, think carefully! Ours is only valid for 31 days abroad (15 days in the USA). So, either limit your trip or plan to change your transportation part way through.
Other contracts have similar intervals. If you want a cell phone, you can buy minutes (expensive), or a 30-day contract of unlimited minutes (La Poste, in France). If you have access to an Orange hotspot in France, you can get 30 days of Wi-Fi.
Airfares are often cheaper if your trip is longer than 2 weeks. But there is also a jump in price if your trip is longer than 30 days! You get a similar jump after 60 days. And sometimes you can't buy a round trip for longer than 90 days.
Just to add simple convenience to the equation, our toiletries all seem to last about a month :-\ . And our packlist doesn't change much for trips longer than a week.
Our bottom line - plan a trip in 30-day intervals...
** Airplane Image courtesy of satit_srihin at FreeDigitalPhotos.net