One of the best things about Apple Search Ads is that it's reacting to bid changes almost instantly. Our recommendations are:
If you just launched your campaign and trying to reach first impressions, wait one day after the creation. Sometimes auctions could act unpredictably during the first day.
If you have 0 impressions, it's okay to change CPT Bid for a SKAG and Default Max CPT Bid for a Discovery every 1 hour.
If you started to have some impressions, it's better to change the bid every 3-4 hours.
If you already have installs, it's better to change the bid once a day.
The best way is to increase bids by 10-20% at a time.