How long is too long for interconnects?

Good advice above. I ideally ALL cables would be as short as possible. But then we'd all be listening to Bose Wave Radios. My suggestions:

1. For unbalanced connections, keep the lowest level connections as short as possible. These would be (in order of lowest to highest) analog source (turntable, tape) to step up, step-up to preamp, preamp to amp, amp to speakers. Thus ICs should be shortest and speaker cables longest. There are exceptions to this rule, or course.

2- For balanced connections, keep source level connections as short as possible. Preamp to amp connections can be as long as needed (within limits). Speaker cables can be short if preferred.

It sounds like you might benefit from using active, powered speakers. Place the speakers where you want and run balanced interconnects to your preamp or high-gain source. Should work great if you haven't yet purchased gear.

for over a decade i had my source and preamp stack near my listening position and amp/s between my speakers. Cables ran nearly 30 feet.  They were usually  unbalanced, with a faraday shield. Nothing fancy.  Noise from those interconnects was never a problem.  I also had balanced cables (both preamp and amp were native balanced - use half or the entire signal), but rarely used them for practical reasons.
I actually think its a **good** idea if it allows you to minimize speaker cables, or place an amp where it gets good ventilation, or some other meaningful benefit. Of course, my cables never cost a gazillion dollars a foot - which with some stuff is a very real consideration.  A few fractions of an ohm and the tiny added capacitance are really of no consequence when the termination is 10-100k ohms, resistive.

I have a strange setup with my integrated amp in a recess behind my wall-mounted TV above the fireplace.  My equipment rack with my digital and analog sources sits in the right corner of the room connected with 5m (16.4ft) rca cables. My tower speakers sit on each side of the fireplace with approx 12 ft speaker cables. I have had zero issues with interference, noise or hum. I was particular when running my interconnects to avoid contact with any power cables which might contribute but I wouldn't be concerned with longer interconnects or speaker cables to a reasonable point.

Long speaker cables are worse than long xlr cables. RCA also has restrictions for maximum length. XLR is the preferred way to hook up all equipment using long lengths and/or your equipment is balanced. Also, very short speaker cables aren’t good either. I remember reading years ago that there is a minimum Length like 4’ or more and then a Recommended maximum length. 

I recommend longer ICs and shorter speaker cables after much trial and error. The best budget ICs are Blue Jean Cables LC-1. Low capacitance, shielded and cold welded connectors. Buy equal lengths (not custom) and try them. You can return them but I still use them for stereo and HT including 15’ runs. Well built and well priced.