I know that this question is too abstract. But. How much time do i need to learn LabVIEW to become average LabVIEW developer? For example, if I buy good book about LabVIEW and have 8 hours per day (on my work) dedicated to LabVIEW learning how many days i will spend on LabVIEW learning? Could you please provide example from your own experience. More information about me that can be helpful: I'm a developer and know c\c++\python and a little bit of java languages.
Answer
Like Swinders said, it might depend a lot on your sensibilities. I have seen people who had a really hard time migrating to the data flow concept. It's a different paradigm from the classic text-based languages and some people can't easily think in these concepts.
If you get past that hurdle, you'll find that the IDE handles a lot of the annoying things you used to take care of for you (things such as syntax and memory allocation). This allows you to become productive very quickly.
It doesn't mean, however, that your level would be high. One potential pit you should try hard to avoid is casting your existing experience onto LV. The most common example is probably local variables. This may be shocking to people coming from a text-based world, but LV does not have variables, per-se. Unfortunately, it does have elements called variables and people migrating from C who find them jump on them and use them as they would use variables in C, leading to LV code which looks like C code and is bad code (at least in LV).
If you do manage to work around this, I would guess you would become better than the global average in less than a month and better than most professional developers after creating three projects you would later look at and say "what the hell was I thinking?".
I never took any of the NI courses (although I understand some of the advanced architecture ones are pretty good), but I would suggest you also spend some time in some of the online communities (such as LAVA or the NI forums) and look at some of the examples and discussions there. There's a lot of material about best practices, design patterns, etc., which would allow you to become a more professional developer.
Above all, do not abandon your current professional conduct. If you have a structured process for designing and developing software, you already have a leg up on the majority of LV programmers. Just make sure you adapt and keep using such a process.