Time to reform is to a first approximation proportional to the rated capacitance, rated voltage, and the length of time that the capacitor has been unpowered. However, there is still a lot of variation. I've had a few 40-year-old capacitors (unused for at least 30 years) that took less than two hours tp reform to 135% of rated voltage, but others of the same ratings that took more than 48 hours.
I usually use a lab power supply with adustable current limiting. I adjust the voltage in increments of 1V, and set the current limit to a different computed limit for each voltage increment so that the maimum power (PS current limit setting * PS voltage setting) doesn't exceed a value believed to have no risk of damage to the capacitor. (This is basically the PDP-1 capacitor reformation procedure devised by Bob Lash, except the voltage increments were 0.5V.) At each voltage increment, the leakage current starts out higher than the PS current limit, so the actual applied voltage doesn't rise to the PS voltage setting immediately. As the oxide grows, the leakage current goes down, so the voltage rises. Once it hits the PS voltage setting, the voltage stays the same, but the leakage current continues to drop. I leave it there until the leakage current stops declining (no further reforming occurring), then go to the next voltage step. With the few capacitors that wouldn't reform, at some voltage setting the leakage current leveled off at a value higher than the maximum rated leakage current. In some cases the leakage current declined some but then started to rise. When thag happens I conclude that thr capacitor has a fault and cannot be reformed. I usually use an lab supply that can be controlled over HP-IB or serial, and use software to automate the process. The software sends a text message to my phone when the process has completed or failed, and logs a fair bit of information to a text file.
