three factors: 1) how much juice does your device take, 2) how much juice does the device claim to be able to supply (continuously), and 3) how much does it ACTUALLY provide?
most devices "want" 500mA (1/2 amp) from USB, and draw far less. (they don't request it, the charger tells the device how much it is
allowed to draw, and that's a one-way message) Most movie phones will want 1 amp and draw the full amount to quick charge until they get near full charge, then drop down to say 250mA to finish topping off. Devices with larger batteries like iPads will pull up to 2 amps if available, and both the device and the charger can get quite warm while bulk charging. If the charger claims to only provide 500mA, then device must either only draw 500, or refuse to charge. Your iPhone etc will pop a message saying "unable to use this charger" if you plug into something that doesn't promise at least 500mA. Other devices will "damn the torpedoes, full speed ahead!" and pull 1amp anyway, causing the charger to overheat or shut down.
It's been my experience that most car chargers "claim" (in paper and electronically) to provide 1 amp, but can't manage it for any length of time (or not at all). These are usually pretty cheap, less than $3 on eBay etc. If you pay more for a charger, the odds of it being able to put out are better. Chargers with multiple USB ports seem more likely to supply higher current. Their larger housing has room for beefier electronics and heat sink.
The same is true for home chargers. There was a serious issue recently with counterfeit iPod/iPhone chargers that were built cheaply and sold cheaply. They overheated immediately when charging an iPad, and some even caught fire. For awhile apple was exchanging those out for genuine iPad charger bricks
for free. (the offer may still stand?)
Even if they can manage the current, most of the cheap ones produce serious "line noise" ("dirty power") when getting near 1 amp output, and that's hard on your electronics. There are some youtube videos illustrating the increase in noise, drop in voltage, and early cutout of cheap, overrated chargers. If your device repeatedly goes into and out of charge mode, your charger is overheating or getting really noisy and your device disconnecting automatically. Find a different charger in that case.
FYI some chargers are rated in watts rather than amps. At 5 volts, 10 watts is required for 1 amp. (watts = volts x amps) Apple's bricks are usually rated at 12w, giving you 10 with room to spare, while providing clean, full-voltage power.
5 port 40 (!) watt home charger:
https://www.youtube.com/watch?v=S7Vx3Jtx7lgEven the cable you use can affect power delivered. Low gauge wire can drop a significant amount of power at higher wattage draw, even causing charging to abort due to low voltage.
https://www.youtube.com/watch?v=doMHmGatyYcAnd someone doing a thorough test comparison with a genuine and a counterfeit apple charger:
https://www.youtube.com/watch?v=_exfmbsPqEIThis last one is the one you really need to watch. The same issues are true for 12v DC chargers.