Britney has 18 months until she's been in the business for 20 years. Though, of course, she's been wavering lately. I feel like she has one/two big hit/s left in her a la Madonna/Mariah.
Anyway, I'll be interested in seeing Beyonce's level of relevance when she's in her 20th year as a solo artist.
Hardly anyone at all. If by relevant you mean enough to be pretty much at the very top. By 20 yrs, I wouldn't even consider Mariah, Celine, and Whitney to be big enough. Certainly, Madonna tho.