It was not always thus - but in recent decades (since about the mid 1980s, perhaps) I have often 'lived with' a particular biography or autobiography for periods of weeks or months - dipping in frequently, and trying to get at the heart of the human subject of the book.
These books (which I am about to list) were not necessarily ones which I would (wearing a critical hat) regard as exceptionally well written books, nor would I necessarily recommend them, and sometimes I would regard them as rather disappointing (the subject was generally more important to me than the style) nonetheless these are indeed biographies with which I spent a lot of time.
The following is incomplete - but in broadly chronological order, or rather the period of my life (some weeks or months) dominated by the book:
Lucky Poet by Hugh MacDiarmid
JRR Tolkien and The Inklings by Humphrey Carpenter
MacDiarmid: a critical biography by Alan Bold
Robert Graves by Martin Seymour Smith
Genius: the life and science of Richard Feynman by James Gleick
Emerson: the Mind on Fire, by RD Richardson (Ralph Waldo Emerson).
The Flowering of New England by Van Wyck Brooks (group biography of early 19th century)
Robert Frost by Jay Parini
Joseph Campbell by Steven and Robin Larsen
Autobiography by John Cowper Powys
Charles Williams by Alice Mary Hadfield
Father Seraphim Rose by Heiromonk Damascene
For example, the Emerson biography by Robert D Richardson is associated with a long period around 1996-8 - I was continually going back and re-reading certain parts, until my copy was fallen into pieces. I then was more easily able to carry pieces of the book to read in bed, the garden or cafes (it is, intact, a very thick book).
My interest in this, as in others listed, was that Emerson seemed (at the time, not now) to have lived a kind of life (not specifically his life - but in some specific respects) which I wanted (in some way) to live - and I think I was trying to learn from this. In fact my motivations were not clear to me, now or since, but anyway I kept returning to the book and trying to puzzle-out something.
Or, and this would apply to the next book - The Flowering of New England, I returned to the parts of these books because they 'cast a spell' on me. VW Brooks prose is incantatory, and evokes a delicious (to me) idyllic quality.
There was, here and elsewhere, an element of day-dreaming, wish fulfillment and escape. The book was working as a technology, or a magical device, to create an alternative world in my mind.
Of course, most people get this from novels - which are, after all, designed to do it. But novels don't usually work for me; at least seldom since my late twenties (for example, the novels of Halldor Laxness did this for me, for a while circa 1999-2001, after visiting Iceland).
So biographies have been, for better or worse, a linking thread of life over the past 25 years.
It may have been for worse - in so far as they were a distraction (on the one hand) yet (on the other hand) apparently held out a (slender) promise of ultimate worldly gratification.
The idea that there had been satisfactory lives, and that these might perhaps be emulated - at least in their satisfactoriness - was a delusion, ultimately.
NB: It is probably not relevant - but I have myself written a (very) mini-biography. A chapter length account of the life of writer/ painter Alasdair Gray - published in The Arts of Alasdair Gray, 1991 - and based on a few months of proper archival research in diaries, letters etc.