The difference between a set amount of time and forever in halacha, is not only quantitative but qualitative as well. "Forever" is a special category in and of itself which is unique from any other duration of time, as long as it may be.
The gemara [Nazir 8b] and Rambam [3/ 11] rule that a person who is a Nazir forever - a נזיר עולם - shaves, brings his korbanos and starts again every thirty days. This is in contrast with a Nazir for a thousand years who goes straight for a thousand years [or however long he lasts] as a Nazir without ever shaving. Even though we KNOW that he will not live a thousand years [so he should be no different than a person who took upon himself a vow of Nezirus forever], nevertheless, there is a qualitative difference between his Nezirus and that of the person who accepted upon himself Nezirus forever. Thus, they have different halachos.
[המדות לחקר ההלכה חלק ב' עמ' שכ"ט]
לזכות ר' שמואל צבי בן ר' דוד עקיבא לברכה והצלחה בכל מעשי ידיו, הוא וכל הנלוים עליו