How long will it take to recover my capital for building a $5000 rig?
Hard to say. Let's say you get the best Cryptonight GPUs, like 5x AMD Vega 56/64 and you learn how to tweak them for best hash and lowest power (so, about 2000-2200MH each card).
Even then, it will depend on the markets, BTC/ETH/LTC price, Monero/Electroneum price, maybe a new high profit coin shows up, maybe it all tanks.
Then it depends on the cost of the parts in your area and the cost of electricity.
Maybe 3 moths. Maybe 12 months. It's hard to say.