at what temp do you start to worry Mark?
Id be afraid to walk away from the hose in case somehow the solution line wiggles out or some kid or animal snags the line ever so gently enough to cause it to start pouring out onto the carpet. Great tip to keep the water flowing in these cold temps.
At 9 degrees yesterday, I felt pretty comfortable for quite some time letting it sit without doing this...like 5 -10 minutes...maybe more, but that is because the hose is heated up...
When it get's below zero and good wind causing windchills, then I am doing this trick....also....I will typically shove it down the hose about a foot so I don't have to worry about it coming out....and with a connector doing free flow, vs a spray nozzle, the suction keeps it in the hose just fine...
We still keep an eye on it, but just an easy way to do this....
Now operating a garden hose feed in this kind of weather gets really difficult, plus finding an outside hook up not very likely....and even those frost free hook ups we have had them being broken when we tap into them....
Homeowners make the mistake of leaving a garden hose hooked up too long, they freeze and then split back inside the home and you cannot see.
Anytime we hook into an outside water spigot we are very leery of what could be waiting for a flood inside the home and you will be blamed for causing it...even though you just avoided them being the first ones to hook up to it in the Spring and flood their home.
This frost free pipe cost me about $1000 or more about 5 years ago....plus ended up doing about $800 worth of work for free too. Home owner was there, asked if we could tap into here outside spigot and we were listening inside for any leaks....sure enough....busted from hose on too long....she freaked as we got water on the floor, 4 ceiling tiles got sprayed and we could have killed her pups in their kennels down there....

(cost was $375 to a plumber to replace and fix, air movers to dry out and time, and broke a
punch bowl she had sitting on the steps as I rounded the corner going downstairs with airmovers....ugh...)
Then she asked me if I thought if she should worry about the spigot on the other side of the house too....Told her YES...WORRY!
But I am not testing it for her today!
So, we avoid tapping into anyone's water source....