Expire? Sequences don't expire!

They can however get near their maxvalue. The upper limit for this is 28 digits long. That's 28 octillion. If you've managed to use all those up... I hate to think how big your data is!

Or - more likely - someone picked a lower maxvalue for ... reasons ...

In which case you're best off increasing the maxvalue. You can find sequences close to the limit by querying *_sequences and comparing maxvalue to last_number:

create sequence s
  maxvalue 500;

select sequence_name, max_value, last_number
from   user_sequences
where  ( max_value - last_number ) < 1000; -- or whatever threshold you deem safe

SEQUENCE_NAME   MAX_VALUE   LAST_NUMBER   
S                         500              1 

alter sequence s
  maxvalue 999999999999999999;
  
select sequence_name, max_value, last_number
from   user_sequences
where  ( max_value - last_number ) < 1000;

no rows selected

You can use the query above to drive the data for an email.