ORIGINALLY it was half and half!
and for a long time it was traditionally about a third vermouth
I like about a sixth - much more than most people these days, but I'm in some ways a traditionalist.
I do also, if they are made correctly, enjoy the Vesper (from Casino Royale, it's the origin of the 'shaken not stirred' quote and it's the ONLY time in any of the Bond books he drinks what you might call a Martini; the rest being movie invention nonsense)
Bond invents it on the spot, and it's sometimes called "The James Bond Martini"
3 parts Gin
1 part potato Vodka
1/2 part Lillet Blanc (sweet vermouth)
shaken, not stirred, served with a twist.
They're great with food, especially spicy food (which covers most of what I cook),
but it's NOT a classic, true Martini
When Vesper dies at the end of the book, he swears to never drink them again.