You've inspired me. I have the same (I think) Doc Johnson 18" double header ... I'm sure I could have left it in for several hours with no problems.
This is the longest object I've left in for hours. I inserted it completely inside me.
Read her blog about inserting Goliath (>5" diameter) all the way to the balls:
https://www.mrhankeystoys.com/blank-rwrlr/goliath
https://www.zity.biz/index.php?mx=blog;ox=showpage;pg=4253;mark=goliath
The longest thing I've inserted is this 1" diameter colon tube in the 66-inch length all the way in:
http://siliconenozzles.com/CT-75French-1-Diameter-Silicone-Serpentine-Colon-Tube-CT-75Fr.htm
The "I'm addicted" review is mine.
The most massive, largest diameter toy I've inserted is 12" of this huge double header, which feels amazing:
https://www.extremerestraints.com/huge-dildos_186/the-python-x-large-double-dong_487.html
I think the spec in the link is wrong. When I put a measuring tape around the widest part of the head, I got 9.5" girth. 9.5/pi = 3" diameter plus a bit. The shaft is a bit under 2.5" diameter.