If you are using an 05, 06, 250, 260, or 450 CPU, there is an Off Delay Timer instruction OFFDTMR. This is an OUTPUT column box and its powerflow status into the box is what drives the actual output. Here's the blurb from the Help system on this instruction:
This IBox will delay turning OFF the specified output by the timer’s preset value. When the input turns ON, the designated output will turn ON. When the input circuit turns OFF, the output will remain ON for the duration of the timer’s preset value, and then the output will turn OFF.
If the input circuit turns ON before the timer expires, the timer will be reset and the output will remain ON.