This all depends on when your A race is for the season and you should work backwards from your A race of the season. If you are racing an Ironman, then 24 weeks before that would be the minimum to start your race-specific training, depending on your fitness level and your experience. If you are racing a 70.3 as an A race, then you could start that race-specific build about 16 weeks out. For an Olympic or Sprint A race you could start your race-specific build up about 12 weeks out.
We like to take the off-season to work on our weaknesses, for example, strength, or swim, bike or run. So, our suggestion is to start the off-season by working on your weaknesses. For example, if your weakest sport is swimming, we suggest working an 8 week swim block into your off-season. (We have a number of plans that can help you do this here
). You may want to repeat that build with an 8 week sport focus for your second best sport as well. Depending on when your A race is you could use these 8 week blocks to build into your Winter Maintenance program or to go straight into your sport-specific program. We are always available to help you line up your season so if you have questions, please contact us at firstname.lastname@example.org