The limiting factor for an attacker using a quantum computer is the number of qubits they can keep entangled long enough to perform a calculation. The qubits required to crack RSA keys are estimated to be 2•bits while ECC is roughly 6•bits, but RSA keys are generally much longer so they end up taking more qubits; roughly 3x on the low end (2048 vs 224) and 10x on the high end (4096 vs 384) see note.
Here's a heavily simplified table comparing the rough number of qubits required to crack common key lengths:
| RSA | ECC |
| Key Length | qubits | Key Length | qubits |
| 1024 | 2048 | 163 | 1000 |
| 2048 | 4096 | 224 | 1300 |
| 3072 | 6144 | 256 | 1500 |
| 4096 | 8192 | 383 | 2300 |
| 15360 | 30720 | 512 | 3000 |
Over the past 20 years, we haven't figured out how to build a fully-fledged quantum computer with more than a handful of qubits. So to compare how much longer it will take a quantum computer to crack an RSA key compared to an "equivalent" ECC key, we basically have to assume a manufacturer will figure out decoherence in a way that can be scaled up. Then it becomes a question of how quickly that scaling will occur.
Assuming linear rates of growth (with 2048 RSA vs 224 ECC low at the end and 4096 RS vs 384 ECC at the high end) RSA gets a ~5-10 year bonus at 500 qubits/year, ~3-5 years at 1K qubits/year, and ~1.5-3 years at 2K qubits/year.
If we assume exponential rates of growth, once we can crack a 224-bit ECC key (the weakest common ECC key) we are one generation away from cracking a 4096-bit RSA key (the strongest common RSA key). At the pace of Moore's law (~2 years) RSA gets ~2 year bonus. Although DWAVE's adiabatic quantum computer isn't useful for these types of problems, they have been doubling their number of qubits roughly every 4 years.
So the difference is actually relatively small: 2-5 years + time to manufacturing breakthrough.
Note 4096 bit RSA and 383 ECC are not directly comparable in terms of symmetric key strength. 4096 bit RSA key is roughly equivalent to a 142 bit symmetric key whereas a 383 bit ECC key is equivalent to a 192 symmetric key. I compare them directly above because they are the largest commonly supported key sizes.