It took me approximatelyn 15 attempts to get the Terra card from Dragon's Neck, 6-7 attempts to get the Bartz card from Battle in the Big Keep, 20-ish attempts to get the Good King Moggle Mog XII card from Thornmarch HM, and I've attempted Shiva about 20 times without seeing a single card drop.

My luck with cards is very weird. I got Gaius on the fifth hand of TT with the Indolent Imperial (which is considered impossibly fast), it took me a good few hours of playing King Elmer to get the Bahamut card, and with the exception of Kan-E-Senna, who still eludes me, I actually managed to get all of the GC Leader cards within 15 minutes of trying for them, oftentimes before the Primal/Scion cards dropped! But at the same time, it took me ages to get Urianger and Minfilia from playing NPCs, I've attempted to acquire Kan-E-Senna and Odin in a rotation, yet have not acquired either despite hours of grinding, and I've bought about 15-20 Gold Packs, only to fail to acquire Zidane or WoL.