We report an experimental investigation of the lifetime of dielectric elastomer actuators (DEAs) under constant DC actuation, with a characterization setup which allows the monitoring of strain and resistance over time for 6 DEAs in parallel. We show how the choice of the electrode influences the lifetime of silicone-based DEAs under constant DC actuation and under different environmental conditions. The DEAs consist of a 12 mu m thick prestretched Elastosil 2030 silicone membrane sandwiched between 1 - 4 mu m thick and 5 mm diameter circular electrodes. Three types of electrodes are used: 1) pad-printed carbon powder - PDMS composite, 2) inkjet-printed carbon black suspension, and 3) carbon black powder. The DEAs are operated under a constant DC electric field in controlled temperature and humidity, ranging from 30% RH at 20 degrees C to 85% RH at 85 degrees C. 6 DEAs were tested simultaneously for each experimental condition (electrode type, % RH, T). Operated at an electric field of 90 V/mu m, at 85 degrees C and at 85% RH, DEAs with pad-printed electrodes show more than 10x higher lifetimes than DEAs with inkjet-printed electrodes. However, no significant difference in lifetime is seen at dry conditions. When operating at electric fields chosen to have the same actuation strain for the different DEAs, those with dry carbon black electrodes show over 5x higher lifetimes compared to DEAs with pad-printed or inkjet-printed DEAs, owing to the lower operational electric field required (50 V/mu m vs. 90 V/mu m) to reach a given strain. At 85 degrees C, increasing the humidity from 20% RH to 85% RH leads to a 40x to 400x decrease in lifetime for DEAs with pad-printed and with inkjet-printed DEAs.