In 2020, practically speaking, how long (time-wise) will a qubit function normally before decoherence kicks in?
On IBM Q, it is a few tens microseconds. The best value of dephasing time T2 is around 500 microseconds. Have a look here IBM Q website and navigate to "Qubits as physical system" and then to "dephasing T2" to see development of T2 in last twenty years.
Nuclear moments of europium ions have been experimentally measured to have coherence times of over six hours, although there is no currently known way of performing controlled logic gate operations on them.