You decide when to make the full and incremental backups and how many backups to keep for that periods count. They are both used to define how many backups to keep. If the period between backups is bigger and the backup to keep stay the same, the backups will be keep for longer.
Check this url where they explain how it works
lets assume daily incremental backups and once month full:
FullPeriod = 30 ( value in days and you may need to tune it to less than 30 if the backup is long, as the day count only starts after the last backup finish and a backup that takes 2 days to finish, you usually only want to wait 28 days to do the backup always at same time of month)
IncrPeriod= 1 (or 0.95 to keep the backup starting at the same hour)
now the how many backups to store... you want 30 incremental backups, so
IncrKeepCnt=30 (30 backups*1 backup a day= 30 backups to store)
For the monthly backup
FullKeepCnt=6 ( 6*30 days)
This is the easy way, the more complex way, lets say you want the same, but weekly full backups:
FullPeriod = 7
So to keep 6 monthly backups (and all full backup weekly for the first month) you need
FullKeepCnt=[4,0,6]
This say that you want 4 backups of the $FullPeriod ( 4 backups for the first 4 weeks), zero backups for the next 2*$FullPeriod (after the 4 first backups, ignore backups for each 2 weeks) and finally keep 6 backups in the 4*$FullPeriod ( 6 backups in the 4*7 days ~1 month apart)