A "lengthy quotation" refers to a quote or statement that is long or takes up a lot of space. It is a way of referring to a quote that is quite extensive or takes a significant amount of time to read or listen to. Full definition
There are too many lengthy quotations from popular hymns, and an irritating misquotation of a rather good little verse beginning «Lord of all pots and pans and needs...» which in its original form rhymes and scans and is quite clever but here is muddled and loses all its charm. (faith.org.uk)
Action: During your manuscript review keep an eye out for lengthy quotations, graphics copied from other sources, song lyrics (watch out for that one!) (bookworks.com)
They aim their text primarily at an academic audience, but they also try to enliven their writing with storytelling and lengthy quotations. (firstthings.com)