Short Hop air time: 38 frames
Fair IASA: 30
Hitbox Active: 4-7
Assuming you only get your 2nd Fair hitbox active for a single frame you have at best a 4 frame window to perform your SHDF.
Mashing for 5 seconds I got like 42 button presses. This is approximately one button press every 8 frames. This is the reason you do not want to mash is because your hang time between mashing will typically be too long to land in the appropriate window. Its much better to time it than to mash.
If you fast fall you were certainly never get your second Fair to come out in time.
Any aerial (save G&W on a few) can be L-canceled. The only time you cannot l-cancel is when the aerial auto-cancels for you. However, this feature of auto-canceling only occurs during the very early frames of an aerial (before hitboxes come out) or at the very tale end of an aerial.
If you timing gets really good try messing around with SH fair-bair, sh fair-dair, SH fair-Nair, and SH Fair-Uair. These are all possible, but have much stricter time windows since they activate a bit later. However, their uses are questionable. If you hit with fair, then Bair/Dair will whiff. Or, if your opponent DIs away on your first Fair, then an Uair will generally miss. In all practicality, just use SH Fair-Fair. However, realize that if your first Fair whiffs or hits a shield your opponent always has enough time between your Fairs to retaliate. Decent punisher, but poor approach.
Source:
http://smashboards.com/threads/complete-marth-hitboxes-and-frame-data.285324/