If you want to buy an eSIM for your travels, you can purchase one whenever you would like. However, once you activate it there will be a specific timer to keep in mind. You should make sensible decisions about your purchase based on this. The expiration date for your data plan will start from when you first activate the eSIM.
If you have any questions, please do not hesitate to contact our Telitravel Support Team. We are here and happy to help!