(Please note that in certain situations,  what ,  when , and  where  can function as relative pronouns.) Relative pronouns introduce relative clauses, which are a type of dependent clause. Relative clauses modify a word, phrase, or idea in the main clause.
The 2014 Survey of Construction (SOC) from the Census Bureau shows that the average completion time of a single-family house is around 7 months, which usually includes around 25 days from authorization to start and another 6 months to finish the construction.
How long did it take to build a pyramid? It could take up to 20 or more years to build In building the pyramids, how did the Egyptians get the heavy/huge blocks of stone in place? Workers used sleds, wooden rollers and levers. They dragged & pushed the huge blocks up ramps of packed rubble to the level they were working on.