Eh, this isn’t unique to our industry.  Everyone in a “knowledge worker” job (Peter Drucker’s term) makes decisions about how best to spend his/her time.  It’s something you usually get better at with experience.  Feedback – knowing how your efforts paid off – is needed to help one make this calculation.

I wish you hadn’t used the term ROI, though.  That concept is over-used in this industry and in marketing in general  Such financial metrics as Net Present Value and Rate of Return are more accurate and relevant to most marketing activities.  For personal time allocation all metrics are hard to nail down; it’s more a matter of keeping in mind that you have a budget of time, energy, and cash, and that you need to get the most out of all your budgets.