its really tough to say , milk bottles were used for some time , I'd personally guess 30's or 40's but you'd need a more detailed dairy book to know for sure
I agree 30's-40's most likely, but a lot of milk bottles are actually dated. Look for a two-digit year on the base (like "35" for 1935 or "42" for 1942 for example). Sometimes it's very small. If that's a Thatcher bottle from the 30's there could be two tiny numbers inside the circle on the base.
Other than that, sometimes you can narrow it down with a maker's mark, or, like RCO said, you just need to know something about the dairy.
Tony, what other marks are on the bottle, usually around the base. I have an excellent milk bottle book of marks that might give you a general idea of date based on those marks.