It depends on your employer in the USA, but I will give you a couple of examples.
At the regional airlines, a first year first officers can expect to make about 20USD per hour with a minimum monthly guarantee of about 70 hours. So a whopping first year monthly salary of 1400USD.
Upgrade is generally seniority based. An upgrade can be expected at most of regional airlines within 5 years. Some shorter, some much longer with the current economy. An average 5 captain would be in the 60USD per hour range with the same 70 hour monthly guarantee...so about 4200USD per month.
Based on this scenario, the first year first officer makes about 35% the monthly guarantee of an average captain. The hourly rate of the first officer will increase per year and by year 5, the first officer rate would be about 65% of the 5th year captain pay.
At the major airlines, you will see the same, but with higher hourly rates. A first year first officer at major will average about 3500USD per month, but with the current economy, the average Captain will be close to top of scale, which is usually 12 year pay. Top of scale for an average narrow body captain at the majors is in the neighborhood of 12,500USD per month. Again, first year co-pilot is about 30% the salary of a average captain at the majors. But just like the regionals, a 12 year co-pilot will make about 65% the salary of a 12 year captain on the same equipment.
The non-scheduled airlines in the USA tend to have a similiar trend, difference is, the non-scheds will hire direct entry captains. So it is possible to have a first year captain salary paired with a 5th year co-pilot. If this is the case, the salaries can be very close.
Hope that helps.