The following chart is a great reference tool for any swimmer. The chart provides general paces in minutes and seconds for set distances from 100 yards (or meters) to a one-hour swim. To use the chart, select a shorter distance (such as 100 yards or meters) and time yourself in a moderately paced swim. Record your time, and use this as a base for swims of longer distances.
For example, a swimmer does 100 yards comfortably in 2:00. Theoretically, this swimmer should be able to do a 200 in 4:00, a 300 in 6:00, etc. If you find you cannot maintain your pace for longer swims, try a modified pace.
This chart is also useful for swimmers who want to do repeats using interval training. One example is five repeats of 100 yards (or 5 x 100's). If you can swim 100 yards comfortably in 1:45 with no rest, you might want to add 10 or 15 seconds rest and do the set of 100's on an interval of 1:55 or 2:00. This interval will allow you to complete the swim, plus get enough rest time to catch your breath before continuing. Repeat interval training is a great way to work toward swimming at a faster pace, which could ultimately benefit your level of fitness.
This chart provides paces between 1:00 and 3:00 per 100 yards or meters -- and it goes up to an hour swim -- but whatever your pace, the chart provides a good foundation for calculating your pace.