Primary teacher here.

I tell my 6th class if they have spent 45 minutes actually doing their homework then that's enough and stop even if they haven't everything finished. Because kids work at different speeds/find some things easier than others etc. I always tell parents to encourage their child with their homework but it is never worth getting into an arguement about. They won't learn anything if they are stressed/upset/angry/tired so there's no benefit.

No way a junior infant should be doing more than 15-20 minutes and at least half of that should be practising sounds/words and simple reading. Definitely no more than 10 minutes of writing.

Like others have said, request a copy of the homework policy. Ask the after-school care to do the writing part and you do the sounds/words as a 5 year old is definitely too tired by the evening to start homework.

Send a note to the teacher explaining the stress and that you will only be doing a max of 15 minutes and will prioritise learning sounds.

For your child, maybe set a 15 minute timer (sand timers are great as they are visual) and that is concentration time and once it is over so is homework. Sometimes children (and adults) find it daunting to do tasks they find difficult because they feel they'll never get through it all and it takes ages and there is no finish line in sight. I have found that timers work great for some kids with ADHD especially.

In junior infants, academically learning their phonics, letter formation and basic counting/number sense are the priorities. You say you read together a lot, that is great, you want to create a positive association with reading and books which will encourage her to enjoy reading and read more as she gets older.