Instead of blood or semen I don’t have money to buy offerings
Food is a pretty standard offering, so yes.
How long does it take for the spirit to consume the energy from the food?
A good rule of thumb is to leave it on the altar for at least 24 hours, and then dispose of it.
…depending on the spirit, if they even ask for an offering, they might accept water, your time, or something personal like a drawing or poem, a story, etc.
I have another tip for you. The offering itself. Thinking of it. Preparing it, is a very nice gesture. And because we throw it away afterwards make the portion a bit smaller. Small size plate. These are still resources which we need to be frugal with.
A candle, coffee with sugar and milk is an offering too in the morning.
Can I just write a story as an offering or does the spirit have to request it?
That’s something you would have to talk to the spirit about. It was just an example. I myself have never offered or been asked for a story.