I have loved you long; I shall love you long: while I live and, if there is mercy, after.
The double “long” stretches feeling into time. For Maximilien, love is duration, not blaze. The line steadies Valentine against the currents in her house. Emotionally, it’s anchor-speech—calm, not flashy. The anaphora gives the promise a heart-beat. In a plot of sudden storms, this is weather you can live under.