You have touched on a surprisingly difficult question we all face. Mostly we intuitively feel our way towards the right answer but it can be very helpful to parse out the influences we factor into our decision.  
  
- **Return on investment.**  
Have you had enough use, in terms of photos and enjoyment, to effectively recover the investment you made in your equipment.  
- **Added value of new investment.**  
In what way will your new investment add value to your photography? Will there be a technical improvement? Will you be more creative? Will new opportunities be opened to you? Will it add more joy to your photography?  
- **Place on the technology curve.**  
While on the steep part of the technology curve it makes sense to have short upgrade cycles with longer upgrade cycles on the shoulder. With mature technology the upgrade cycle is determined by things like wear and tear.  
- **New needs and aspirations.**  
Have you grown as a photographer and found that you have new needs or aspirations that can only be satisfied with new equipment.  
- **Wear and tear.**  
Has your equipment suffered from significant wear and tear requiring replacement?  
- **External influences.**  
Are you the victim of external influences? Companies bring very considerable powers of persuasion to bear in the hope of shortening your replacement cycle. Camera forums add to this problem, creating a veritable hot house of eager anticipation of the latest and greatest.  
- **Changed perceptions.**  
Have your perceptions of your equipment changed? Is this because you have found the equipment to have shortcomings? Or is it because you have allowed yourself to become the victim of external influences?  
- **Disposable income.**  
Are you sure you are using your disposable income in the wisest way, considering the many demands on you?  
- **Reasons for original investment.**  
It is helpful to cast your mind back and ask why you made your choice in the first place? Do those reasons still apply? If not, what has changed?  
- **Law of unintended consequences.**  
Have you taken the time to list possible adverse consequences? Believe it or not this is what corporations get wrong every time. Their hunger for the marshmallow blinds them to the consequences, see next point.  
- **Delayed gratification or the [Marshmallow Test][1].**  
You have read through this list and by now you should have a good idea of what your decision should be. Now ask yourself a searching question. Are you able to [practice self discipline and wait][2] until the time is right to re-invest in your equipment or have you been overcome by your desire for a marshmallow? Are you like one of the kids that failed [the Marshmallow Test][3]?  
  
My advice is to carefully think about each of these items and ask the question, does this support my decision to re-invest? There is no magic formula but you will have examined the problem from all angles making it likely that you will make the right decision.

It all comes down to one last question. Will this really make me a better photographer? Is it worth the cost?


  [1]: http://www.ted.com/talks/joachim_de_posada_says_don_t_eat_the_marshmallow_yet.html
  [2]: http://en.wikipedia.org/wiki/Deferred_gratification
  [3]: http://www.ted.com/talks/joachim_de_posada_says_don_t_eat_the_marshmallow_yet.html