Final answer:
The information provided does not specify how long William Cullen Bryant worked on "Thanatopsis"; instead, it discusses works by Phillis Wheatley and William Carlos Williams.
Step-by-step explanation:
The exact duration William Cullen Bryant worked on "Thanatopsis" is not detailed in the information provided. "Thanatopsis" is a poem by William Cullen Bryant that contemplates death, and while the creation timeline of the poem could be relevant, the material given does not specify the length of time it took Bryant to write it.
The references provided relay information about a different set of literary works, particularly those connected to Phillis Wheatley and William Carlos Williams, rather than any details pertaining to Bryant's work on "Thanatopsis".