modelling dynamic cycles

The exercise is to model a recurring cost cycle. In this example there are some light bulbs need replacing periodically based on their usage. I cannot get beyond the first cycle or at least not in a dynamic way e.g. when the bulbs are replaced in y2 the next replacement occurs in y4 etc and although it might sound simple it is not because when the operating hours change the interval of the recurring costs changes too. Anyway, I have attached a spreadsheet with a basic model with minimal moving parts in it and some help would be very much appreciated...since I am stuck. Thanks!