A good rule of thumb is to have 25x your annual expenses saved to retire. Take your average monthly expenditures and multiply them by 12 to get your annual costs. Once you have that number, multiply it by 25 (the average number of years a person is retired) to see the base number you’ll need to have in retirement savings.
While you’re most likely planning to live more than 25 years as a retired person, this number is only the base and doesn’t account for the interest you’ll accrue. Most FIREd people live off their interest alone, shooting for around 4% withdrawal every month.