Set Image CacheValidity timespan in XAML

I have an image in my XAML and I'd like to set a longer timespan for the CacheValidity.

<Image Source="{Binding PictureUrl}" CacheValidity="5" />

I could do this in code by binding to a TimeSpan on my ViewModel, but I'd rather just define a timespan straight in the XAML.

So, how do I write a timespan of 5 days in XAML?