Can I descend during downwind or I can only descend when I am at base?
See Chapters 7 and 8 of the FAA's Airplane Flying Handbook.  

https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/airplane_handbook/media/10_afh_ch8.pdf

The descent point for the approach is going to be largely based upon the handling characteristics of the specific type or model of aircraft you are flying in.  In general it's commonly taught to pilots that you begin your descent from the traffic pattern to land when you are on the downwind leg and abeam the threshold of the runway you are going to land on.  You will select a descent power setting at this point, configure the aircraft for landing and begin the descent.  When you turn base or final is going to be up to your discretion but in general should be no more than 1-2 miles from the airport.  This turning point may also be given to you by ATC at a towered airport for aircraft spacing e.g. "Cessna 1 2 3 X-ray yankee, extend your downwind; I'll call your base."  You may also have to do the same thing at "beehives" or busy, non-towered airports for traffic spacing in the pattern as well.