That's not an easy Q to answer. Some houses have merge points AT the house entrance (H4, TrT, SoE, DE, CG). Others have merge points that have a built in post-merge wait of 5-10 min (i.e. Stranger Things, Scary Tales, Slaughter, Poltergeist, Blumhouse), so there's that. Even if a house is a walk on, it's a 5 min walk through the whole queue and another 5 from the exit back. So a walk-on house can be 10 min of walking, not counting the house itself.
My rule of thumb is that if you see a 5-10 min standby, don't even waste the Express. 5-10 min is probably a walk on. Sometimes 15 min is a walk on. There is nothing more pathetic than seeing two people in Express when the event opens and standby is a walk on. Those two people either don't realize they won't be able to Express a second time, or are too drunk to care.
In general, Express is about 25%-30% of standby. But that is a big generality. I've done a (rare) walk-on using Express when line is clearly 90 min, and I've done a (fairly rare) 25 min Express when standby is 30.
I will do standby if it's 20 min or less. Otherwise, Express. When I was younger 30 min was the threshold. Conversely, if I see a large wait (i.e. 100 min), I usually skip the house until it's 60 min because Express would be too long of a real wait for me. Waiting in an express line for 45 min can only piss a person off. So for you, maybe 30-75 min standby is a great time for Express. Up to you. You are paying $10+ for the privilege of skipping one line, for one house, one time, so make the most of it.
It also depends on how the employee meters between Express and standby. I've seen them equally split and heavily weight in one way or the other. Affects the waits drastically.