“Big time” usually means “a lot,” but a pun has been cited in print since at least 2015:
 
“I let my friend borrow my grandfather clock. He owes me big time.”
 
     
Wikipedia: Longcase clock
A longcase clock, also tall-case clock, floor clock, or grandfather clock, is a tall, freestanding, weight-driven pendulum clock with the pendulum held inside the tower or waist of the case. Clocks of this style are commonly 1.8–2.4 metres (6–8 feet) tall. The case often features elaborately carved ornamentation on the hood (or bonnet), which surrounds and frames the dial, or clock face. The English clockmaker William Clement is credited with the development of this form in 1670. Until the early 20th century, pendulum clocks were the world’s most accurate timekeeping technology, and longcase clocks, due to their superior accuracy, served as time standards for households and businesses. Today they are kept mainly for their decorative and antique value.
       
Google Books
Gagged and Bound:
A book of puns, one-liners and dad jokes
By Nick Jones
Full Media Ltd.
2015
Pg. 63:
I let my friend borrow my grandfather clock. He owes me big time.
 
Twitter
Steven David
@thestevendavid
My friend has borrowed my grandfather clock. He owes me big time.
11:01 AM - 28 Feb 2018
   
Twitter
Teknoman
@Andrew_D28
I lent my grandfather clock to my friend. - reallyfunnyshortjokes: He owes me big time.
1:05 PM - 22 Mar 2018
 
Twitter
LaffGaff
@LaffGaff
I lent my grandfather clock to my friend.
He owes me big time.
#funny #jokes #jokeoftheday
6:12 PM - 22 Mar 2018
 
Reddit—Oneliners
I lent my grandfather clock to my friend and now he owes me big time.
submitted March 26, 2018 by madazzahatter
 
Reddit—Cleanjokes
I lent my grandfather clock to my friend… (self.cleanjokes)
submitted March 26, 2018 by madazzahatterQuality “Ugh” Contributor
He owes me big time…