Forerunner will save data on up to 5000 laps. In the US, most people use a lap distance of 1.0 miles. If you run an average of 48 miles (or less) per week, Forerunner's memory will hold at least two years worth of data.
For mapping / graphing purposes, Forerunner 201 saves additional data points for just your last 10 runs. Forerunner 301 appears to save these details much longer. Data can be saved on a PC for as long as you want by uploading it into Training Center.