hhdl

The BofA card's XP are just like any other XP. They'll roll over as and when XP from flying or other sources roll over. They may help you reach a status level (if that level is Gold or Platinum, the credit card XP would net decrease your rollover, though you'd have a higher status (and the soft landing would still apply: qualifying for Gold means at least being Silver in the year following and qualifying for Platinum means at least being Gold in the year following (and Silver the year after that): the combination of rollover elite credit and a soft landing is, as far as I can tell, completely unique to Flying Blue); if that level is Silver, the credit card XP will likely increase your rollover, since your rollover without the card would be zero).

In the long and extra-long J round trip (66 XP each way) case with the BofA card's automatic 20 XP per year for continuing to carry the card, starting from Silver with zero rollover:

Year 1: start with 0 XP, get 152 XP from flights + card, requalify for Silver
Year 2: roll over 52 XP, get 152 XP from flights + card, move up to Gold
Year 3: roll over 24 XP, get 152 XP from flights + card, move down to Silver
Year 4: roll over 76 XP, get 152 XP from flights + card, move up to Gold
Year 5: roll over 48 XP, get 152 XP from flights + card, requalify for Gold
Year 6: roll over 20 XP, get 152 XP from flights + card, move down to Silver
Year 7: roll over 72 XP, get 152 XP from flights + card, move up to Gold
Year 8: roll over 44 XP, get 152 XP from flights + card, requalify for Gold
Year 9: roll over 16 XP, get 152 XP from flights + card, move down to Silver
and so forth

If you earn 100+x XP (x being positive and less than 80, of course) per year consistently, the proportion of time you spend as Gold will over the long run be x/80: 140 XP will bounce between Silver with 40 XP rolled over and Gold with zero rollover, so 152 XP per year would be Gold 13 years out of 20.

Hitting the $15k per year spend target (I strongly advise trying to do it in no more than 11 months and then sock-drawering, to avoid possible calendar mismatch issues etc.) for the 40 extra (60 total) XP annually would mean 192 XP per year, which would mean Gold 9 years out of 10 and the other year would be Platinum.

Disclaimer for the pedantic: this assumes no future program changes... having a 10 or 20 year plan for status is probably not the smartest idea one could have.