longest feasible rca run

Longer runs the sound starts to get tamed.
Lows lose the speed of attack, highs taper off some. It is not a big deal for your purposes.
50 ft would be ok.
naturally if you could use balanced line that problem would disappear. But RCa will be OK.
Using ONE 50' length, not several stuck together would be way better.

A lot depends on the voltage/drive capability of the equipment at the source, and the sensitivity(output vs voltage in) of the driven end. Then there is the issue of noise sources(RFI/EMI) along the distance(the primary reason for the use of balanced cables, in those applications).

i love balanced cables!