How long RG-6u i can use in my cctv? what's is the effective range using RG-6u?
Rg6/U is rated up to a max of 1000-1500 feet. If you need longer you can use Rg11 which is up to 3000ft.
You'll also need power. (They do make hybrid cable that has RG6/u and 2 x 18 Gauge wires for power.)
Make sure to ensure that the proper power is supplied on a long run. You can use this calculator.
First you have to determine how much you can degrade the signal by and still find it acceptable. This depends upon the noise content in the original signal.
Then looking at the band you need below you can read the attenuation expected for 100' and calculate the distance form that. SInce this is CCTV (Baseband) I'd use the 10 MHz band as a first estimate.
Band Attenuation per 100ft for RG-6