Nacho:
Good job, keep at it and you'll get there just fine. It takes time, just like anything else.
Once you achieve the full-lotus, for even a brief second; it takes time to maintain it for the duration. Just keep stretching and opening the joints. The muscle memory is there, but it will take time to get them used to it again - after so many years.
Concerning your original question, there is no special or better time for your stretching. Your body will be the best judge of that; and, veryone is different. Work slowly and according to your schedule. Try and sit in the position whenever your are seated, to watch TV, or sit and read, or sit with company. Some people swear by certain times for stretching; but, again, your body is the best judge: listen to it.
Iam2, with time you'll be able to do it too. But, please, take it SLOW! Better have that knee checked, or at least be cautious and observe good form and exercise protocol when stretching. Master Lin's suggestions in Level 2 is excellent and most effective. Forget the prestel type stretching and work up to your ability: "Rome wasn't built in a day," and you won't become fully flexible over night - either. But, slowly and consistently and you will ARRIVE!
Much Love ....
