It depends on the policy type: classic annual travel policies cover any number of trips, but each individual trip only for 42 to 70 days. If you are away longer – world trip, sabbatical, work & travel, studying abroad or emigration – you need long-term international health insurance: depending on the provider, it applies for 12, 24 or up to 60 months, often with an extension option from abroad.
Two deadlines are decisive: first, the policy generally has to be taken out before departure – only a few special policies insure people already abroad. Second, coverage usually ends with your return to Germany; plan your re-entry into statutory or private health insurance (keyword: entitlement option) in good time. All variants, benefits and costs are in the guide Long-Term International Health Insurance.