It depends on the device, cable, charger.
USB port on PC when on is 250-500ma max.
USB port on PC with a lightening bolt next to it is 1 amp.
Many USB chargers are usually 1 amp.
iPad, Qualcomm quick charge 2.0 are > 2 amp.
Your device will draw whatever it needs.
iPhone - 1 amp
iPad - 2.4 amp
Nexus 5 - 1.5 amp
Qualcomm quick charge 15-30 watts total @ various voltages (5,9,12v)
However... the kick is also the cable! For Apple devices, always use MFi certified cables. The cheap ones, iPads will complain about. For USB, Anker USB cables are the best I found and handle 2 amps reliably. However, depending on your USB charger you may need a charge only USB cable. PortaPow was inconsistent. A charge only adapter or shorting the Anker USB data pins with aluminum foil was better.
For example, my Nexus 5 will usually get 1 amp. With a short cheap USB charge only cable connected to the 12watt iPad charger, it gets 1.5 amp 70% of the time. It wasn't until I bought the Anker USB cables that it ALWAYS gets 1.5 amp, but only from iPad 12 watt, QUALCOMM quick chargers, Scoche 12 watt dual car charger. Also my Nexus 5 only does 1.5 amp when the battery is between 15%-70%. This is true for many devices, where at the bottom or top end, charging is not at max.
For safety, only buy chargers that are UL rated. They tend to be well built, heat up less and reliably charge more consistent at the higher rates. Duracell and G.E. chargers are nearly always UL rated. Duracell you'll find at Big Lots, Staples. G.E. you'll find in the electrical surge protector section at Target, Walmart.
TLDNR. Use iPad 12 watt or QUALCOMM Quick Charge 2.0 charger. Use Apple MFi or Anker USB cables. Charge only USB cables are great for travelling since there is not chance of any data exchange.