How long do webhooks take to be called?

Hi folks

I've been setting up a website integration with Zoho Subscriptions which fires a webhook when a subscription is created.

Up until earlier today, the webhook seemed to be called instantly on creation of a subscription.

However during testing this afternoon we discovered what appeared to be a problem... the webhook did not appear to be being called at all.

A few hours later and some of them now seem to have completed.

Is this normal? If so, how long should I expect a webhook to be queued before it is called?

Many thanks!