Question: Given a battery with a certain capacity **C** (in Ah) and given a certain charging current **Ich** (in A), how I can calculate the charging time?

Answer: 
Charging time depends on a very large number of factors and so capacity and current are insufficient information to make a good calculation. For example, for a typical lead-acid battery you don't charge beyond 75% capacity at a constant current.

You can trade off battery lifetime against charging time. The appropriate charging current isn't arbitrary, A 100 A charging current won't charge a battery 1000 times faster than a 100 mA charging current, in part because heating the battery affects the charging process and capacity.

![enter image description here](https://i.stack.imgur.com/04gJH.jpg)  

*From [Battery University](http://batteryuniversity.com/learn/article/charging_the_lead_acid_battery)*

Many people do use a rule of thumb for lead-acid batteries that assume 80% charging efficiency and simply divide capacity in amp-hours by charging current in amps and add about 20%. Realistic charging efficiencies can vary considerably.

See also [Peukert's law](http://en.wikipedia.org/wiki/Peukert%27s_law).

For other types of battery chemistry, different rules apply.
Divide C by Ich.
Example: C = 20Ah, I=1A => 20h

Also, charging current might be given as a ratio of C. In this case it would be marked as 0.5C.

Edit: Manufacturers give a C rating = capacity in amp-hours. That says nothing about the actual charge or discharge current. Then they might give a ratio of C for charging and a different figure for discharging.

For example, RC LiPo batteries might have 35C discharge rate and 2C charge rate. So if you have a 2000mAh battery you can discharge it at 70A and charge it at 4A.

For lead-acid ones you typically get crank current (~15C) for a few seconds, normal operation discharge current (might be 1C) and charge current (typically 0.1C).