Simple residential blower control
If the timer relay you're talking about is a customer level product made to be plugged into the wall, I would trust it for a 20W fan. But, if you're making your own thing, I would use a Triac or Thyristor instead of a relay. The difference is thyristors are make to only turn off when their current is around 0, so you do not need to worry about inductive backlash. On the down side, this gives you less control of timing.