How long does diphtheria vaccination last for?

Boosters needed. The official recommendation is that one should receive a tetanus-diphtheria adult booster once every ten years to keep up immunity to both diseases.
Usually a life time. The diptheria toxoid is very immunogenic so it's immunity lasts a lifetime. Diptheria is combined with tetanus and pertussis. The tetanus component needs to be given every 5-10 years.