What should be the gap between EB-5 I-526 approval and the DS-260 process?

U.S. CIS will not likely approve your I-526 petition in one year. After U.S. CIS approves your I-526 petition, you can delay proceeding with the DS-260 and the immigrant visa process. You can proceed with the immigrant visa process after your commitments abroad are completed.

Completing the process within 3 years is optimistic - you can always delay to 4 years or more, but based on present processing times, 3 years would be an optimistic guess.

You can pursue consular processing at a delayed pace as needed. If the primary goal is to obtain U.S. residency, it may make more sense to pursue consular processing after the I-526 petition is approved without delay and after receiving your immigrant visa (green card), apply for a reentry permit which will allow you to reside abroad for up to 2 years consecutively as a green card holder. You can apply for a reentry permit multiple times. If you are trying to delay the residency process for tax/financial reasons, you can take consular processing at a slower pace so long as you keep your NVC case active once a year.

It would be a miracle to see an I-526 approved in one year. Look at their current processing times. Once approved, we can gauge the consular processing time more accurately depending on what consulate is selected.