Hi Puurkova,
those who don't have perfect pitch will almost not be able to hit the first note correctly without any pitch reference. For most theremin players a pitch preview is very helpful. It allows to hear the theremin's pitch in an earpiece while the main audio output is still muted through the left hand/volume loop. But even a pitch preview is useless if you don't have an intro (from the accompaniment) which allows you to find the starting tone.
That's why I am asked from time to time to install still an additional output on a theremin which gives a shaped and buffered pitch preview signal. This can be used to connect a visual chromatic tuner (I recommend the Korg DT4) which shows clearly the note (C, C#, D, etc.) and has a sophisticated system of red and green LEDs around to show you clearly the precision, if you are exactly in tune or a little sharp or flat. In order to avoid interference with the pitch and volume field, this tuner should be mounted below the theremin, for example with a clip on the mic stand, so that you can rapidly see where you are.
Here is an example setup which I built for one of my students : https://www.facebook.com/photo.php?fbid=3688935198952&l=c6b3a7e167
Your second question "How long will it approximately take..." has a simple answer : It depends.
Clara Rockmore, the "grande dame" of the theremin said "it takes many years of study and hard work" and "Before you begin, you should know where you will go". This is for sure right if you intend to play the "classic" theremin repertoire at a professional level. The Dutch thereminist Thorwald Jorgensen for example was already a professional musician (percussionist) when he decided to play the theremin. He worked for about 5 years, 4 hours daily, before he gave his first concert.
Since theremin teachers are most times not "round the corner", most theremin players are more or less autodidacts. There is a big risk of taking "bad" habits in your playing technique which will from a certain point on prevent you from making further progress. Thus it is very important to meet an experienced theremin teacher from time who will observe your playing and give you hints how to improve your technique. There are many occasions to do so in Europe: Lydia Kavina organizes Theremin lessons for beginners and masterclasses in Oxford/GB about every three or four months, then there is a series of theremin gatherings round the year:
- January: The N/O/D/E festival in Lausanne, Switzerland (Carolina Eyck/D and Thierry Frenkel/F)
- April: The theremin spring academy in Leipzig, Germany (Carolina Eyck/D and Thierry Frenkel/F)
- July: The theremin summer academy in Colmar, France (Carolina Eyck/D, Wilco Botermans/NL and Thierry Frenkel/F)
- November: The "Without Touch" festival in Lippstadt, Germany (Lydia Kavina/GB, Carolina Eyck/D, Wilco Botermans/NL and Thierry Frenkel/F)
It can also be interesting to follow these people on Facebook. Lydia and Carolina announce all their concerts in Europe and overseas and they are always ready to meet theremin students in the respective places.