People Core Services/Audience Library - Real Time Audiences , How Long is "Profile" Valid For

If I created an Audience in the People Core Audience library using an Analytics Dimension, how long is the "lookback" period or profile lifetime (to use Target terminology)? That is, if Dimension X came to be associated with Visitor Y, how long would Visitor Y be placed in the Real-Time Audiences built using Dimension X?

Thanks.