Expect to spend about 30 minutes every other week doing partial water changes, cleaning the tank, etc. It is not that much. Just to compare with the small dogs require average one hour a day maintenance, hamsters- several minutes a day, rabbits- 20 minutes a day, horses ans ponies- about half an hour a day (a lot). The pure ratio- maintenance vs enjoyment time of the fish thanks is quite high. You will have enough time to look at your tank and its inhabitants.

Be prepared to spend several hours researching the hobby before you make your first purchase. Go to several pet stores to find one that looks like a reputable place. Get some beginner books. Read this beginner FAQ several times.

Most people who get frustrated with fish tanks made mistakes that could have been easily avoided. The way to avoid mistakes is to learn the basics (e.g., the nitrogen cycle) BEFORE you put fish in your tank. There are few things more upsetting than frantically reading the FAQ for the first time, while three feet away your beloved fish are dying. Remember: most aquarium problems are easy to prevent, but hard to deal with after the fact.