Hi Shaun,
Thanks for your reply (and for have read my mental masturbation..LOL).
Totally agree with you, for SOP, not for dT.

Probably the equation could be developed as non-linear but I want to make the things simple so try to express a simple linear differential equation.

Totally agree with you for the coefficients, L (liking) is the most important and worth 60/70% of the weight IMHO, then E (rarity or exclusivity) while dC and SOP are few percent...

Instead I give an high importance to dT (or in any way we want to talk it), I feel a great joy after weeks that I don't wear a watch but after two o three days I need to swap it...my dream would be to have all my watches at home with me to swap them every day, but for safety reasons I cannot so swap them in rotation leaving almost all of them in bank...

An interesting improvement that would make the equation non-linear could be to express the time-dependent term as an exponential decay: instead of considering the term c3*dT (that is the satisfaction S is linearly dependent on which amount of time is passed since the last time you wore the watch) could be more adherent to the reality to consider this term like:

C3*e^(-Cw*t_worn/Cu*t_unworn)

Remember that C3 is the weight that you give to satisfaction S depending only on the time you wear the watch, without considering the other factors.
While Cw and Cu are respectively the weight (importance) you give to the time you wear the watch t_worn and the weight you give the time you don't wear it.

In this way you will have the maximum satisfaction (equal to C3) in the first moment you wear it, then the satisfaction will change depending on how much time you wear it compared to how much time you do not you wear it.
We can set the initial condition t_worn = t_unworn = 0 with liking "undefined" (in effect until you will not try the watch on your wrist you will never know how much you like or unlike it … LOL).

In real life many processes follow this exponential decay law, for further information see Wikipedia here:
http://en.wikipedia.org/wiki/Exponential_decay

This means that larger decay constants (x = t_worn / t_unworn) make the quantity (that is the satisfaction S) vanish much more rapidly.
This is in agreement with what I usually feel, that is if I will wear often a watch (t_worn >> t_unworn) I will get bored sooner than the opposite case.

Summarizing, the satisfaction in wearing a vintage watch can be expressed, in my opinion, as:

S = C1*E + C2*dC + C3*e^(-Cw*t_worn/Cu*t_unworn) + C4*L + C5*SOP

I hope I have not bored you all...

All my best.

Gianluca