The normal lead length to still obtain a satisfactory solder fillet is 1mm. This
does depend on the pad size as the joint will tent to form a ball if the lead is
short and the pad large. On a plated through hole the pad size is not as
significant as a single sided board on reliability.
As the lead size becomes longer the incidence of solder balls increases. This is
regardless of thickness of the PCB. The lead length should not be longer than
3mm as then we start to have a less than optimum solder wave st up.
You may be interested in the SMART Group training video on "Conventional and
Surface Mount Design"

Bob Willis
SMART Group 
Tel 01245 351502
Fax 01224 496123