A 270mAH hour cell can in theory deliver:270mAH for 1 hour27mAH for 10 hours2.7mAH for 100 hours27A for 1/10 hour (=6mins)270A for 1/100 hour (=0.6mins = 36 sec)in theory, but in practice cells are not efficient at high curents and other things impose lower limits (like the thickness of metal in the cells internal construction/wiring.The amount of current a cell can deliver for 1 hour is known as the C rating.The standard charge current for most cells is usually 1/10th C which in your case is 27mA or 0.027A it's the same thing. It theory it should take 10 hours to charge your cell at 1/10th C but because the charge process is also inefficient it normally takes about 14 hours.Fast charging is usually defined as anything over about 1C or 2C eg 270mA (=0.27A) or 540mA (=0.54A) in your case.If your cells are designed for fast charging then you should be ok at 0.5A and possibly more.At 2C your cells should take about 1/2 hour to charge. At 3C its 1/3rd hour = 20mins etc