Parlay insurance is almost always issued in the form of a promotional offer, meaning the amount you're eligible to claim back on a losing parlay depends on the event, sport, or tournament the promo is available for.
Most commonly parlay insurance comes as a percentage of your wager, typically around 25-50%, but sometimes up to 100%. FanDuel, like all other betting sites, does impose some limitations, like minimum odds, wager amount, or maximum refund, but this is typically around the $25 mark so it's still quite valuable.