The need to modernise school physics to encompass the fundamentals of Einsteinian physics is widely recognised. While the ability of students to comprehend qualitative Einsteinian concepts has been demonstrated, little information exists to determine the best student age and the duration of instruction required for introducing Einsteinian physics concepts. Here, we compare 1-day excursion-based interventions with longer 10- and 20-lesson interventions spread over 3 and 10 weeks, respectively. The programmes covered similar materials with quantitative evaluations for students in year 7 and 9 (13-15 years old). While short-duration interventions are shown to be adequate for introducing core concepts, the longer-duration interventions led to significantly improved uptake of derived concepts. Differences in uptake according to academic talent and gender were observed, particularly for derived concepts. The students tested had generally positive attitudes to science, which changed little during the interventions. The results provide valuable information for introducing Einsteinian physics at schools.