The volatility is the problem as it makes calculations unreliable but a year to cover your costs on a 2 year (if you upgrade to the latest graphics card every generation) or 4 year (if you go every second gen) life is a reasonable investment.

It's literally a ROI of 100-300%.

Or to put it another way it's similar to a risky stock with a compounding yearly return of 41% for four years.

Using a graphics card for mining that you have anyway is just gravy (apart from externalities which are another big problem).