It has been raining since a long time. (correct the sentence)

In English grammar, 'since' and 'for' are prepositions used to indicate time, particularly with perfect and perfect continuous tenses, but they have distinct uses:

    
• For: Used to express a duration of time. It answers the question "How long?". For example, "for three hours," "for five years," "for two days."
    
• Since: Used to indicate a starting point in time. It answers the question "From when?". For example, "since Monday," "since 2010," "since morning."

In the original sentence, "two days" refers to a duration, not a specific starting point. Therefore, 'for' is the correct preposition to use.

He has been ill since last Friday.

He has been ill since Friday last.

It has been raining since two hours.

= It has been raining for two hours.

It has been raining ‍since morning.