In a typical tank, you can draw off somewhere between 70-80% of the stored water before you notice it is cooling off. On a 40-gallon tank, say 30-gallons. Now, because it is typically hotter than you would use in a shower, and depending on whether it is winter or summer, you might use 75% hot, and 25% cold, so you're back to maybe 40-gallons of shower temp water. A typical showerhead uses 2-2.5 gpm, so maybe 16-minutes. A typical teenager will easily exhaust the WH, and depending on the recovery rate, most people would then have to wait.
Depending on the type of washing machine, and the cycle you use, some don't use any hot water, and if warm, it could vary from 20-gallons to maybe only 3-4 (top loader on warm verses a front-loader).
One way to make a tank 'look' bigger, is to raise the storage temperature. Then, you normally mix in more cold to get the desired temp. But, it is more dangerous. Where I live, ALL WH need a tempering valve, that limits the outlet temperature of the WH, regardless of how hot you make it in the tank, tempered water comes out (can be adjusted, but most come preset to 120-degrees F). If you have the flexibility, sometimes people will tap off the hot water before the tempering valve to feed things like the washing machine and dishwasher, where that hotter water can make a difference, but the rest of the house only gets the 'safer' cooler water.