After flying in to Ninoy Aquino International Airport, Manila, how long should I plan to take using public transportation (bus?) to get to General Santos?
Most of these flights are on so small airlines that it's hard to get historical performance data on but the Philippine 453 flight out of 34 observations, 85% was on time, averaging only 9 minutes of delay, maximum 31 minutes of delay. So yes, the schedule is pretty firm.
Rome2rio does show some bus connections but it's 1.5-2 days and does not show a significant savings over a flight taking less than 2 hours so I have not pursued this avenue further.
On further research I have found some outdated information on ferries going from Manila to Iloilo and on to General Santos taking even longer closer to three days. I haven't researched this either but the Wikipedia page on 2GO reports it to be a modern fleet so one can presume it's on schedule.