First, communicate to the client that it's taking so long (and why its taking so long). They will ask for an estimate, and you can defer the estimate for a day

First, communicate to the client that it's taking so long (and why its taking so long). They will ask for an estimate, and you can defer the estimate for a day or two. I bill my clients for 5 minute chats too - but for this one I would not. Second, put a hard limit on the billable hours for such a thing. 30 hours, 40 hours, whatever you decide. Now you are in a better position to tell your clients their bill-estimate, but not a time estimate. If the problem is too hard, you could spend 100 hours in the week to solve it and only bill 40 hours - Might feel like being undervalued, but does not let the "guilt" build up. This also forces you to find workarounds to the problem because you are losing money now. Third, hire somebody else at your own expense even if there are zero or slightly negative margins. I've done this in domains that I am not an expert. The thing is, you should be the go-to-guy(or go-to-company) for your client. However, if it's a frequent issue while working or don't love the challenge - you might consider moving out of such a domain.