Sitemap generation time

I've noticed a change in my 'Sitemap Generator History'
Before it was taking:-
49.37 s => 454,425 URLs
and now it takes
86,414.28 s => 491,026 URLs

any reason it would suddenly take so long to generate? (is been like that for the last few days)?

Looks like sitemap generation is interrupted unfinished and it resumes the generation on next day when scheduled task is executed again (hence 24 hours for the "total time from start to finish").
You can increase max_execution_time setting in PHP configuration to resolve that.