When to use Database

I can serialize my data to harddisk. How can I know it is time to use a database to store my data?

All sorts of possible reasons:
- you get fed up of maintaining your serialisers
- the data is accessed by more than one program (say back office update and web-facing query written in PHP)
- the data is accessed by more than one machine (or machine architecture)
- the data becomes so large that simple files no longer scale efficiently
- the data has lots of internal relationships (eg customers and orders)
- you find yourself writing ad-hoc query languages to search your data

Also keep in mind that "a database" doesn't necessarily mean a separate entity on your network. If you're not familiar with embedded databases like Berkeley DB or Sqlite, take a look. They're a very lightweight way to get many of the benefits Salem mentioned without a lot of extra set up.