I'll say enough when I've followed my current plan and seen no result.
I spent 18 months researching agents in the sf&f field, while I was writing and polishing my latest project.
There are probably 160+ agents who market fantasy in the U.S., the U.K., Canada, and Australia, but many of them aren't open to unsolicited queries.
Since I can't afford to pitch my work directly to them at conventions and workshops, they're off the list.
I follow 82 agents through their online presences and industry reports. I read books published by their authors, so I get a sense of what they like.
30 of them are considered top-tier, with recent Publishers Marketplace 'nice deals' listed for sales in epic, heroic, and traditional adult fantasy. Some of them only consider YA epic fantasy, not adult, but I've kept them in the list because I might tweak my adult fantasy back to YA levels. Another 30 to 35 agents are middle-tier, with midlist sales and/or a preference for urban fantasy and paranormal romance over traditional fantasy. Some want science-fiction only, not fantasy. The rest are agents who've claimed an interest in fantasy on their blogs or in interviews, but haven't made reputable sales to one of the big traditional publishers, or one of the well-reviewed smaller publishers. Or they've sold work to a publisher which takes unagented submissions.
I've just now started to query the top agents on my list. I've sent out six queries in the last month. So far, I've had two form rejection letters and one personalized letter with some useful comments. I'm going slowly, so I have the chance to tweak my query letter. I won't send out any more queries until the last one on my list responds, or three months have gone by.
If I have no takers at all in that upper bracket, then I'll know I need to refocus on the query. I'm writing on other projects in the meantime, so I have more to offer later. I will look at the second batch of agents only if they show some really great sales in the meantime. I'm not sure I will consider the tail end of the list at all.
Filigree