How can you tell how many SL days equal RL days?

[ShortCake Windstorm]

It depends on whether you are talking about clock time or sidereal (sun) time.

Clock time is the same as Real Life.  Second Life runs on Pacific Time, since LL's headquarters is in San Francisco CA.

Sun time can vary, since a region (or in some cases, a parcel) owner can set a custom time of day or a custom day cycle.  However, the "standard" SL day cycle is four hours long, with three hours of daylight and one hour of night.

For more on time in Second Life, see my blog posts:

"Does Anybody Really Know What Time It Is?"

"Sidereal (Sun) Time in Second Life - The Environment Editor"