It is best to wait a few days between hpts. This gives your hormones a chance to change between tests. Aside from this, the question of exactly how long has both economic and emotional components.
Can you afford to take an hpt every day? The less you can afford the hpt, the longer you should wait between testing.
Which is harder for you emotionally - the wait or a bfn? If you have a big emotional reaction to a bfn, then wait longer. But if the wait puts you in a worse emotional state, then test sooner.