Tl;dr: mosquitoes are good at learning, but they’re unlikely to become attracted to deet outside of laboratory environments in which their response to it is directly being tested, because they go a long time between feeding and each formulation they encounter is more likely to be different. Additionally, it was difficult to get them to feed the first time in the presence of deet, so the risk is probably highest when the repellent is wearing off, meaning you should reapply following the instructions on the product.
Tl;dr: mosquitoes are good at learning, but they’re unlikely to become attracted to deet outside of laboratory environments in which their response to it is directly being tested, because they go a long time between feeding and each formulation they encounter is more likely to be different. Additionally, it was difficult to get them to feed the first time in the presence of deet, so the risk is probably highest when the repellent is wearing off, meaning you should reapply following the instructions on the product.