There is a famous book "Playing the Piano for Pleasue" by Charls Cooke. Firerside NY 1960 which gives advice on maintaining and improving piano skills in adults.

Cooke advises 30 min practice a day - and no more- BUT it must be done EVERY day -even on holidays. eg find a piano o rkeyboard somewhere   hotel, studio hire,,piano shop , private house..

As a sign of improvement one knows oneself -by smoothness, opinions of others , and lack of necessity to correct "fractures" or weak points in the pieces you are working on. Cooke also suggests only have about 6 pieces in practice. as an amateur pianist who has followed this regime I can tell you it works. It is the greatest motivation i know and feeling yourself improve makes you want to continue.

so many people who learn piano as children give it up later, even if they reach quite high levels in the standard music education systems.

as a quantitative standard the Suzuki books can indicate progress as of course can the standard examinations. biu the subjective feeling is more important for lasting enthusiasm for playing.

Hi James,

I was looking for something newer and more scientific tests. But I will take it in count if I do not find anything else .

One more thing, I forgot to put in the question what kind of musicians I'm studying. So, the study is related with musicians that play wood wind instruments with a single reed, eg, saxophonists and clarinetists.

And, of course, I appreciate your help.

Hi Ferran. 

In the UK, the Associated Board of the Royal Schools of Music (ABRSM) has set some standard tests to evaluate a musician progression. The exams differ depending on the instrument. For pianist for instance you have at least 8 grades that refer to different skill levels. Please check the links for more information. 

Hope this helps.

Kind regards.

http://en.wikipedia.org/wiki/ABRSM

http://us.abrsm.org/en/our-exams/piano

Thanks for the help Joao