How long should I give an estate agent before switching?

Our house has been on the market for 6 weeks with a local independent agent. We've had 4 viewings, no offers. The asking price is based on their valuation (£320k) and comparable sold prices in the area.

The agent says it's "just the market" but their marketing photos are pretty average and they haven't suggested any changes to the listing. Starting to wonder if we need a better agent.

How long is a reasonable period before switching? And do online agents actually work?