During his stay in Paris for about a year

[Bassim]

Is my sentence grammatically correct?

During his stay in Paris for about a year, Peter had an opportunity to meet some of the greatest painters of our time, which according to him, had the crucial importance in his development as an artist.

 

[Tarheel]

Is my sentence grammatically correct?

During his stay in Paris for about a year, Peter had an opportunity to meet some of the greatest painters of our time, which according to him, was crucial in his development as an artist.

See above.

 

[Roman55]

I would say, his stay of about a year. He stayed for about a year, but his stay was of about a year.