What is the meaning of the phrase "contended even unto death for seven long years?"

Hassock's mansion was built between 1854-56 and was a 'station' on the 'Underground Railroad' which helped escaped slaves to freedom. The wikipedia entry for Hassock says that during the period when he built his house he 'deepened his connection with the Underground Railroad' which suggests that the abolitionist group in Ottawa, Illinois was in existence before 1854.

It is quite reasonable to imagine that they had been actively assisting escaped slaves from 1853 which would match Hassock's statement that the group had been risking severe consequences (possibly even being killed) for seven years before 1860.

and contended even unto death for seven long years.

and contended = And have been argued and fought over

even unto death = even to the extent that people have been killed

for seven long years. = for seven years that have been so difficult for everyone that each year appeared to be longer than a normal year.

Why seven years? Does this number have some significance?

Yes it indicates the length of time (at that point) that the arguments and fighting have been continuing.

“We hold these truths to be self-evident, that all men are created equal, that they are endowed by their creator with certain inalienable rights, that among these are life, liberty, and the pursuit of happiness ...”

This is the most famous passage of the Declaration of Independence, written in 1776 to justify a rebellion against the British Crown; that rebellion (“contention unto death”) resulted in the Treaty of Paris seven years later.