Guys, speaking of round-trip low-latencies, I've just listed an RME FF400 on Ebay.  I've accumulated too many interfaces, and it's just sitting in the closet, so the RME lost the flip of the coin (Actually, I figured that would likely be the easiest to sell).
 
www.ebay.com/itm/272225150783
 
Because it's not cosmetically perfect, I'm letting it go on the low-ish side.  So if you've ever desired an RME interface, but couldn't justify the $1K+ price tag, here's your chance to get a good deal on one.  Also, no worries if your PC lacks firewire -- I'm throwing in a SIIG PCIe Firewire card (TI chipset), which has always worked great with all of my FW devices.

I've done more than my fair share of gigs where someone's bass amp is 20 feet away from me and vocal/acoustic instruments only in the foldback. Keeping in time becomes as much a matter of watching them as listening to them at that range. This being in a band with no drums, so no-one in the middle of it all to act as a fixed reference point.

I can't handle bass or guitar at much over 10ms latency, my timing just collapses and eveything feels wrong when I feel the fingers hit the string then the note sounds after a distinct gap. Walking out among the audience at the end of a 150' cable might have worked for Albert Collins, but not me.

Why I love playing live with in ears now,,, even in my solo act, and why I do most tracking with phones on or sitting less than 2' away from the speakers. 
And why I track with most efxs disabled and never in a million years monitoring the back end of my interface. 
My system is about 7 to 9 ms RTL but because I never listen that way it's a none issue. 

I move around a lot when playing...
Tried a set of non molded IEMs.  Every time I'd move... they'd pop out of place.
 
Without IEMs, on larger stages it's easy to get more than 20' from your monitor.
If you roam over to the guitar player and your not in his/her monitor, it can be harder to hear yourself... and then you're dealing with significant latency.
The advantage to that much physical space: Far less prone to feedback issues

Because I'm always on the mike, I've gone with a hardwired iem system. I removed the right ear bud from my Shure SE215's so I only use one side. This I find gives me the best of both worlds. With both buds in I find I'm too disconnected with the stage sound which after 40 years is just to weird to deal with. 
In our band this also puts it on the side with the drums and bass so they are attenuated. I can still hear my guitar amp in my right ear the way I've always heard it. 
 I use a Rolls PM 50  which is on my mike stand. My vocal mike can be made as loud as I can stand it,   and then I blend in the monitor mix. Perfect. Total control finally. As I say, even in my solo act this tightens up my timing as I'm hearing my backing tracks bang on in my left ear. I can also stay in touch with the audience if they come up and talk to me. 
 So once again,,,, latency is not intuitive to a good performance.  In the studio or on stage. Long live IEM and headphones... 

I like where this thread is going, because some have chimed in with real world examples, like how far you are from your speakers before you lose your "lock" with yourself or your bandmates. If you do some quick math, sound travels about 1 foot per millisecond (700mph x 5280 ÷ 3600 seconds in an hour ÷1000ms=1.02). So ask yourself how far away from an amp can you play before you start to feel the lag? 10ft?  20 ft? That's how many ms of latency you're feeling. When listening to monitors , how far away are you? factor that into your RTL. So if you are , say 8ft and 20ms RTL ,you are really feeling 20 ms RTL + 8. If 20 is the"magic " number where yo really start experiencing it, you should be shooting for 5-10 ms. if you can hit this sweet spot, you're in good shape. Less than 5ms RTL will most likely be imperceptible.