There are a lot of methods, one of the simpliest is using OB1_PREV_CYCLE variable (temp variable in OB1)

A #Pump_works

JCN notw

L #Stored_time // DINT type (or TIME type, which is the same)

L #OB1_PREV_CYCLE

+D

T #Stored_time // Your time contains working time in ms. If you divide it by 3600*1000 - you get hours

notw:NOP 0

....

Please mention - it would have some inaccuracy, but think it doesn't matter for such a task. Also you can just configure OB3x with similar code. To be more precise - use SFC64 in similar way or store starting time by means of SFC1 on starting pump, do it again on stopping and compute the difference (in this case you have to worry about syncronization).