Giving subjects feedback may affect their performance over time because of learning effects. The choice of giving feedback may be far less trivial than subjects getting bored yes or no. And quite frankly, subjects will get bored after a 100 trials, let alone 500, feedback or no feedback...
You have to be aware that subjects can show learning effects in speech tests (De Jong et al, in press). I am unaware of the specific task you give them, but perhaps below answer will help you make a rational decision on this.
- Procedural learning affects the way people perform because they may get better in the task, simply because they have to learn how the task works ('Oh wait, the trial starts already!' 'Oh wait, where was that answer button again?' 'Oh wait, what was the green button for again?'). If subjects show procedural learning effects, they will get better over time. This is oftentimes regardless of whether you give any feedback; subjects simply need to 'get the hang of it'.
- Conceptual learning will affect the performance too. This type of learning is strongly affected by whether you give them feedback. For example, if your subjects are unfamiliar with the languages and you are not providing them feedback then they will never learn them. On the other hand, when for example they hear words from a closed set and each word is repeatedly presented, and the experimenter provides the correct answer ('This trial you heard senorita and that is Spanish and means female' or whatever) and the subjects repeatedly hears this word then for sure at a certain point they will learn from feedback that it is Spanish. However, without any feedback they will never learn it.
Even feedback after 5 minutes may affect their performance ('Oh I'm doing great, I can probably relax a bit'). In my opinion, subjects should never be given feedback, not until, perhaps, after the entire experiment is over.
- De Jong et al., Ear & Hearing, in press