It seems the default is 5 minutes:

internal sealed partial class DefaultHybridCache : HybridCache
{
    internal const int DefaultExpirationMinutes = 5;

Source: https://github.com/dotnet/extensions/blob/eac5c605634171d870d8454cc82ff7cf4780a8f4/src/Libraries/Microsoft.Extensions.Caching.Hybrid/Internal/DefaultHybridCache.cs#L27