Il y a belle lurette (que) is a quaint, and somewhat poetic, expression that means: in a long time, in ages. It also comes as ça fait belle lurette (que) or depuis belle lurette, all with the same meaning. Although it is quite similar to the French depuis longtemps, it tends to add emphasis to the matter at hand. For this reason, it is often followed by an exclamation mark.
In French, the literal meaning is il y a une belle petite heure, which translates as a good little hour ago. This may not sound like very long, but as a matter of fact, it is. The hour in question is a metaphor for "some time;" and is certainly longer than 60 minutes. That period of time doesn't need to have been particularly good either. It might have been weeks of gloomy weather, months of misery, or any mix of good and bad hair days. The expression doesn't inform on the quality of the elapsed time. What it tells is that it sure feels like a long time. The word belle is merely used for emphasis here; it adds considerable length to what would otherwise be, well, some time.
Lurette by itself doesn't mean anything in French. It is an alteration overtime of the word heurette, which is a diminutive for heure and means little hour.
The expression appeared in the 19th century as il y a belle heurette. The silent H became an L over time because of the L sound in belle, in what linguists call a process of "agglutination." As to when heurette became hurette, no one knows exactly, but local dialects and regional accents were known to account for such variations in vowel sounds in the past, before radio and television established a norm.
Je n'ai pas parlé français depuis belle lurette!
It has been a long time since I last spoke French!
Il y a belle lurette que je ne l'ai pas vu!
I haven't seen him in ages!