My game quota is now limited to, say, 1.25 games/week
I think your rate of improvement will depend for example on the amount of time you spend analyzing your games.
Remember tham IM Martin suggested the 15 minutes of intense training/day would be enough for a steady improvement (I think he suggested 1ELO/week).
My experience from my earlier life as an improving player was that improvement happened in "leaps" but the waiting between them was the tough part. I believe that continuously thinking about the game in a serious way will garantuee improvement over time.
There must be zillions of clever ways to measure improvement. I think it is important to trust your own "feel" about the game. You might feel that you are improving even though that is does not show much in, say, rating.
A private "Hall of Shame" might be useful. A collection of you own blunders. The idea is basically to review your categorical blunders and to make sure that you don not "reuse them".
It might also help you selfimage of your improvement: "What?? How on earth could I miss that?"
Lol, nice thought! Why not complement with a real "Hall of fame" too where you keep the games you're pleased with - in order to regain some self-confidence after reveiewing the blunders 
.
Thanks all for your comments guys, appreciated!
PS: Tonight, during my chess lesson, my coach will propose me something "new" for me: the introduction of 5 minutes game to develop reflexes and lower blunders. It sounds really weird to me, but I will keep you posted!
It is a well known truth that, for people at my level, the two key things to improve are:
1) tacticts
2) slow games, followed by analysis of the mistakes done on both sides
At the moment, I am managing a regular daily tactics routine, but I cannot do anything more than a couple of long games a week. Does this mean that I will never improve, at this rate?
Any thoughts, highly appreciated in advance!
Andrea