### Count merged cells in calc

I am using a simple time tracker for tasks in calc (libreoffice 3.5.3.2). The task names are entered from a list, and the cells are merged for continuous durations.
In order to calculate the total time per task, I need to know a count of rows spanned by each task. A simple COUNTIF() would only return the number of occurrences of the task string, whereas I am looking for a metric that counts the number of cells in the merged ones.

I searched a lot, but cannot figure out how to implement this. I know it is not advised to use merged cells for calculations etc. but having to enter text for every entry makes it more tedious.

Thank you for any input!