I've noticed on various JLPT publications that particular levels have an associated recommended study time:

Level 1: 900 hours
Level 2: 600 hours
Level 3: 300 hours
Level 4: 150 hours

These seem quite optimistic to me. For example Level 2 requires knowledge of about 1000 kanji (and not just their Heisig keywords) plus a vocab of about 6000 words plus proficiency in several hundred points of grammar. Not to mention upper intermediate listening skills. If anyone can achieve that in just 600 hours of total study, I'd be quite impressed.

Any thoughts on what would be more realistic study times?

I think those estimates are a load of bullocks, pardon my French.
They may apply for Chinese (and some Korean) people who can already read most kanji and only need to learn everything else, but for people like us, I think a more realistic estimate would be those numbers multiplied by 3 or 4.

It has been a while since I looked at the JLPT guides, but as I recall these used to be clearly stated as minimum recommend classroom hours. My evening school teachers at the time certainly would not spend classroom hours on general learning of kanji, and they would also expect that home study to classroom study was at a ratio of about 5:1 or more.

I passed level-2 with less than 600 classroom hours of Japanese study, but my own investment of time would have been far beyond that.

At the end of the day, they are just estimates to help people who are coming to the JLPT without any knowledge of the exam gauge what level they can attempt.