问题:

I know the Google Translate API has a limit of 5,000 characters per request, but how many requests are allowed under a certain period of time?

回答1:

It seems to be 100,000.

Though this can be increased. (Refer discussion in the link)

回答2:

With the (paid) google translate API v2, one can set different quotas, like a Per-User Limit, which throttles the maximum number of characters a user can send for translation / second. You can set this as low as you want, i.e. 10.0 characters/second/user.

Remark There is no more free google translate API: V1 was shut down on Dec. 1, 2011, and for V2 you have to pay. See: Google language APIs

回答3:

By default, it's 5 requests/second/user and 200,000 requests/day (Billable limit) but you can increase the limit requests/second/user in your console web developer google (at https://code.google.com/apis/console/).

In the sidebar on the left, select APIs & auth, then select APIs and select Translate API.
On the API's info page, select the Quota link near the API name.
Click the Set per-user limits... button to increase your limits.

IMPORTANT : if you don't see the line "BigQuery API" -> 5 requests/second/user is because you don't have a professional account gmail. Also : You have to enable Biling (professional account or not).