Hi,
I'm not experienced with PICs, but it will be similar to other microcontrollers.
The capture function uses a timer/counter and captures it's value at the capture event.
So the resolution and the max time depends on the timer/counter setup. Especially the counter input clock frequency, prescaler and counter bit width.
Example: Input frequency 8MHz, prescaler 1024, 16 bit counter.
Max time = 2^16 * 1024 / 8MHz = about 8s.
If you need more than 20s you could increase prescaler value, decrease input frequency, increase bitwidth.
If this way 20V is not possible, then it is an easy interrupt task to count every counter overflow.
For every timer overflow just add 65536 to your timing value ( for a 16 bit counter).
Klaus