The משנה says that if someone says they are making one big נזירות, or they are making a נזירות from now till the end of the world, they are a נזיר for only 30 days. The reason for this is a מחלוקת ראשונים, which is explained at length in the גרי"ז הלכות נזירות, פּרק ג' הל' י"א. תוספות in ד"ה הריני explains that the person never meant to say he wanted to be a נזיר forever. Rather, he is just saying that נזירות to him is very long and as if it is taking forever. Even when he said from now till the end of the world he just meant it feels really long or else he would have specified 500 years (which is how long it takes to walk the world). The רא"ש sounds like we just explain his words that way since סתם נזירות להקל, but it's not clear what he means. The רמב"ם has a very different פּשט. The רמב"ם in הלכות נזירות פּרק ג הל׳ א says that if a person says they are accepting a big נזירות its only for thirty days since he didn’t mention a specific amount of time. In הלכה י"א he says if the person says they are a נזיר forever, then they are a “נזיר עולם” (who can be מקיל בתער annually). If they say they are a נזיר for a particular amount of time, even 200 years, then they are a full נזיר for 200 years (with no hair cutting). The רמב"ם seems to hold that the main thing is whether you mentioned a specific time. If you did, you are a full נזיר for that time with no ability to be מקיל בתער. If you did not give a finite time but just said “a lot” then its just 30 days. If you said “forever” with no further specificity, then it’s a נזיר עולם where you cut your hair if it's too long annually. תוספות and the רא"ש would seem to disagree and hold that if we knew you meant what you said and you wanted it to last a long time, it would, and you would stay a full נזיר until the time you meant had for sure past. תוספות doesn’t hold that mentioning a specific time is critical at all. What’s a little harder to understand is the רמב"ם in הלכה י"ז who says that if you made yourself a נזיר like “this whole house” or “this whole box”, then we ask the person what they meant. If they say they meant “a long time” then its 30 days. If they say they meant “סתם” then we assume the house was full of mustard seeds and he is a נזיר עולם where you are מקיל בתער. Apparently the רמב"ם understood that saying a full house is just another way of saying “נזיר עולם”.

The simple reading of the גמרא, at least the first part, sounds like תוספות since the גמרא asks “but he said he was a נזיר forever so why is it only 30 days”, to which the גמרא says he didn’t mean it. According to the רמב"ם even if he did mean it, it would be only 30 days! The Brisker Rav explains that the רמב"ם will understand the גמרא’s question to be why don’t we think he meant to declare himself a נזיר עולם.

This is a fundamental מחלוקת as according to the רמב"ם regular נזירות by its definition must include a specified amount of days or else it’s 30 days and תוספות and the רא"ש would hold it is not necessary to specify a time.