Dec 2014 started xolair once a month for hives, are there any effects if i would stop all together or if i pushed this month back 2 weeks?

Unlikely: There may be breakthrough hives, but it is more likely that there won't be. Many patients tolerate longer intervals between treatments. If you stop altogether, then the hives are expected to return (depending on their cause if known and how long you were treated prior to starting Xolair). Discuss with the prescribing physician. If a vacation is scheduled, also discuss plan for breakthrough hives