Sometimes I hear people say something like the example below. Is it correct to use the word since in this manner?
Mosquitoes are surviving on earth since millions of years.
Sometimes I hear people say something like the example below. Is it correct to use the word since in this manner?
Mosquitoes are surviving on earth since millions of years.
No, this is not correct. Since is employed to designate a timespan running from a definite† point in time up to the current 'Reference Time', whatever that may be:
Bob has lived here since 1992. ... The timespan runs from 1987 to the present.
In 1874 Wagner completed Der Ring des Nibelungen, which he had been working on since 1848. ... The timespan runs from 1848 to 1874.
If you want to define the timespan by its duration you should use for:
Bob has lived here for twenty-two years.
Wagner worked on the Ring for twenty-six years.
Mosquitoes have survived on Earth for millions of years.
† This does not mean that the point must be defined with great precision. For instance, “Since the first millennium BCE” is acceptable.
In your example sentence, for should be used in place of since.
Since is used when you are stating the start time(date/month/year/century etc.) of a duration in your sentence.
eg.
He has been playing since five O'clock in the morning
I have been working here since 1991.
This festival has been going on since January.
For is used when you are using the duration only in your sentence:
eg.
Mosquitoes have survived on earth for millions of years
He has been playing for five hours straight.
I have been working here for 10 years now.
This festival has been going on for past 2 months