Depends on the definition of serious. Lots of 3-4 month relationships, one 15 months when I was 19 and one 15 months when I was 24. Next longest was DH - we've been together 7.5 years and married for 5.
Didn't expect or want to marry first long term boyfriend. Expected but didn't want to marry other long term boyfriend. So I guess if serious is defined as being with someone who I both expected and wanted to marry, then one.