Also what is normal learning curve in time to learn 8051 core in years?
what do you mean about "learning"? never in doubt?, sometimes in doubt?
When I started with the '51 20 years ago, I was constantly "in doubt" now I am "in doubt" a few times a year and screw up, because I should have been "in doubt" and was not, about once every two.

why should I learn assembler first before C code and when should I use both?
The assembly code gives you an insight in the workings of the bugger that C never will. Knowledge of the underlying assembly is essential to writing efficient C. An experiment I once conducted with rewriting something in "efficient C" that was written as "true C" by someone else showed enormous gains in reduced code size and faster throughput. Classic examples are the use of malloc, function pointers, ints where char will do .....

As to using both: use C when it suffices, assembler when it does not. I have some functions that are too slow in C running on a 100MHz one clocker, in assembly they are 3 times faster. The basic gain comes from things like knowing that a movx @Ri will not cross a page boundary, C can not "have that knowledge". For plain vanilla operations in internal RAM there is not much gain using assembler.

Erik