Based on a few legality guidelines read on the DocuSign support pages and elsewhere, electronic signatures are as valid as hand written signatures under a variety of circumstances, hence I would assume they are valid as long as hand written signatures. However, I have found nothing that specifically stated X number of years. I would recommend checking with a legal expert or lawyer to first verify electronic signatures can be used for your Trust fund documents....and if so then I assume there is no expiration of the electronic signature. DocuSign is the tool for the application of electronic signature, I do not believe they will guide you on your specific scenario.