At what point does a new agent with no sales raise red flags? When they have been agenting for two months? Six months? A year? I know that not all beginning agents make sales quickly, but when does not having any sales become a huge red flag to someone considering them for representation?
This is a question with multiple answers. There are multiple answers because "new agent" can be applied to people in wildly differing situations.
Someone who hung out her shingle in Left Of Nowhere, Nova Dakota last year cause she loves authors and wants to help them is a far different cry from someone who worked at BigAssAndDonchewFergitIt House as an editor for twenty years and is now turning to the Dark Side.
New agents come in all levels of expertise and experience.
The more expertise and experience, the sooner they can make sales (generally speaking.)
If someone who's worked in publishing for 20 years isn't making sales within a year, I'd wonder.
If someone who's never worked in publishing and is agenting part time to "help writers" EVER makes a sale, I'd be astonished.
Here's the real rule: I don't want to be first on a new venture.
If a publisher is just starting out, or someone has a great idea for new idea about movies you can watch on your phone, I want to be their third project, not the first. I want them to make all those inevitable mistakes with some other guy, not me.
This applies generally to agents as well. I'm not always well received when I say "anyone can sell a book" but it's very close to true. Selling isn't the hard part of agenting. The hard part is keeping writers well-published. To do that you need years of experience.
Of course, when I was first starting out I had a very different pitch on what you wanted in an agent, and experience was not on the list I can assure you.
To answer your question: anyone who is agenting should have made a sale in a year. Anything less than that and they're not full-time, or not well-connected, or not getting good projects. All of those things are Not. Good.