Obviously it'll depend a bit on map size and such, but the simple answer is "whenever you have enough excess Happiness to handle it". The big difference from earlier Civ games is that you no longer want to place cities just to fill in gaps; you're perfectly okay leaving a bad stretch of land vacant. That being said, you should be expanding quickly; not as fast as in Civ 4, but still fast. If your happiness is at +10ish (or lower, if the target location will add a luxury) then go for it.
If you can place a city near a luxury resource you don't have, do it, period. (Even if you DO have that resource already, it's often worth it once you start trading with other empires.) The five happiness you'll gain is worth any cost, and it's one fewer luxury for your enemies.
If you can place a city near a strategic resource you're in short supply for, do it. Especially if it's Coal, which you WILL use for Factories. (Invariably, there'll be some massive arctic deposits of coal and oil that'll require an ice town.)
And keep an eye on apparently empty tracts of land; invariably these end up being where the Coal, Oil, and Uranium spawn. In my last game, three of the world's six coal deposits were within a single city-state's borders, which led to a MASSIVE world war once we all reached Industrial; up until that point it had seemed like a worthless ally to make, but once the coal popped...
If you try sticking at a small number of cities, you'll be fine in the short term, but in the long term you'll lose the resource race. You NEED at least one deposit of every strategic, and as many unique luxuries as possible, just to survive in the late game. Having multiples of luxuries is a huge boost to diplomacy; I've effectively bribed most of the AIs in my last game into friendship by just giving them excess Dyes and Silks (since I had 5 of each) for tiny amounts of money.
One thing to keep in mind is that cities now use three rings of tiles instead of two. So you really don't want your cities to be too close together. Conversely, the new spread mechanism makes the actual positioning of good tiles more important; in Civ 4, as long as the good tiles were somewhere in the 21 it was all about the same, but in C5 the ones in the first ring are far more valuable than those in the third, and you don't have to worry as much about including a few "worthless" tiles in the radius or overlapping at the wrong point with one of your other cities.
Do NOT worry about the +30% cost for social policies per city. It's an additive change, not multiplicative, so it's really only a problem if you suddenly annex a bunch of unproductive cities at once. That said, I HAVE had a few times where I delayed using a Settler for a turn or two just to grab a new SP.
Connect new cities to your capital ASAP, and they'll pay for themselves in trade route income. This is where the Harbor comes in; if you place a city far from your capital, don't bother with a road unless it's necessary militarily, just build (or rush) a Harbor. Once it's connected, the money will get nice very quickly. (Just remember that trade income depends on city size, so it'll take a bit to get going.)
Now, there's one thing to watch out for: unit upkeep. In Civ 4 and earlier, you'd have one military unit per city and usually one worker per city as well. In Civ 5, the cost for units is prohibitive, and this includes Workers. If you find yourself telling workers to sleep until a border expands or something, disband them.
There's a slight diminishing return on costs; the first few units are cheaper, so in a small empire, it won't be so bad, but as your empire gets larger this becomes a real problem. So if you find yourself having economic problems in the late game, start disbanding nonessential units, especially including Workers. (In my last game, Small map, I had 23 workers in the modern era, each of which cost me 6 gold per turn in upkeep; it added up FAST, so I disbanded 15 of them.)
(The key phrase to keep in mind: "Up or Out". Every unit should either be upgraded ASAP to the leading edge, or disbanded. Anything else will kill you in upkeep. So if a unit has less than 4 promotions, don't bother keeping it around once it's obsolete unless you absolutely have to.)
