So how long do the Cs have to stay under the 2nd apron to avoid repeater penalties? Until what date?
There are different repeater penalties. The basic repeater penalty is the extra dollar per dollar over the tax level in pure tax payments. A non-repeater team pays $1.50 for every dollar over the tax level up to $5 million; a repeater pays $2.50 in that first bracket. At all levels it is an extra $1 for a repeater. The second apron has nothing to do with that type of repeater penalty - it applies to any team that has been over the tax in three of the last four years. As of now, the Celtics are on their fourth straight year over the tax. They cannot avoid the repeater category this year, but obviously if they get below the tax line, then there is no penalty because it only adds to a tax bill, and if you have no tax bill, there is no repeater penalty.
If they drop below the tax this year, but then go above the tax level next year, they will still pay a repeater penalty for 26-27. To avoid future repeater penalties, they need to stay below the tax for a second year. If it is this season and next, then they would be clear of the repeater penalty for the following two years. (Or if they drop below in 25-26, go above in 26-27, and drop below in 27-28, then they would pay a repeater penalty in 26-27, but be clear of the repeater penalty in 28-29.)
There is also a penalty for multiple second apron appearances. From what I understand, if a team is above the second apron in 3 of 5 years, then its next draft pick falls to the end of the first round regardless of record. I believe the second apron only came into existence last summer, so the Celtics have one second apron appearance against them (24-25). But if they go above the second apron in 2 of the next 4 years (including this one), they will get that first round pick penalty.