rough idea

a rough idea (about sth /of sth) (a general idea; an estimate) — приблизительное представление о чем-л.; представление в общих чертах

Example:
The following composite dialog, in which every answer is a lie, is typical of the sort of thing we endured:
Me: How many days a week do you work?
Employee: Five.
Me: Any overtime?
Employee: Almost never. We get time and a half in pay for overtime. Me: How much do you make per hour?
Employee: I don't know.
Me: How much did you get for your most recent pay period?
Employee: I can't remember.
Me: Rough idea?
(Ну а хотя бы приблизительно? А если приблизительно?)
Employee: I can't remember.
Me: How do you deal with the fumes from the glue?
Employee: It's no problem. We have masks.
(Washington Monthly, "Confessions of a Sweatshop Inspector", 2008)

get / have a rough idea — получить / иметь общее представление, приблизительно, в общих чертах представлять себе что-л.

Example 1: I need to get a rough idea of how many people will be there. — Мне нужно хотя бы в общих чертах представлять, сколько там соберется народу.

Example 2: If you've been in a Starbucks, you've got a rough idea of what's coming.

Example 3: He had seen no windows since he and Breen left the Council Chambers, which meant he had only a rough idea of where he was. (V. E. Mitchell, "Imbalance")

Example 4: Theorists propose, for example, that a kind of pressure exerted by the zero-temperature vacuum keeps quarks and gluons cooped up as protons and neutrons. But the scientists have only a rough idea of how it works. (Science News, "Seeking the Mother of All Matter")

give sb a rough idea — дать общее представление; обрисовать картину в общих чертах, приблизительно

Example 1: Can you give us a rough idea of the average number of visitors?

Example 2: Can you give me a rough idea of the cost? (= tell me approximately how much it will cost)

Example 2: A close examination of your monthly water bills (outside of the irrigation season) can give you a rough idea of how much graywater your household produces. (Mother Earth News)

Example 3: By analogy, detailing all the laws that define a government gives only a rough idea of how it works in practice. (Fortune)