There is no default. A common advice is to use tracks a little bit wider and with a bit more clearance then your PCB manufacturer's minimum specification. It's also quite common for PCB manufacturers to have different rules for different PCB's. 2-layer PCB's often have coarser rules then 4 or more layers. If your use too narrow tracks, then the quote for your PCB costs may be higher without you realizing it.
But for a number. 0.25mm is pretty wide and compatible with (nearly) any PCB manufacturer. It's a good starting value for "beginners" If your PCB's get more complex or you want to route smaller PCB's, then it's easy to go narrower. With KiCad for example (only EDA suite I know) you can create a selection of tracks and then change width / clearance of the tracks, and when you make them narrower later, you can just push them aside more withe the interactive router. Going the other way is much more time consuming, as widening the tracks instantly leads to hundreds or even thousands of DRC violations because the minimum clearances are not met.
And there is also current handling. A 0.25mm wide track can already handle 800mA (with 10 degree celcius temperature rise) This is plenty for all signal wires (usually <10mA DC) In principle it would also be enough for routing power to most of a PCB, but none the less, for power I tend to use at least 1.5mm, even if it's just to get power to an I2C EEPROM and does not need 3A.