I look at it a 2 months should be plenty of time for 2 x12 transaction types, 1 inbound & 1 outbound.
Your transaction definitions shouldn't change by trading partner. Your rules around the transaction are what gets affected.
Once the transactions are defined and the initial code/mappings are defined you should be able to address unique issues for variations in less than a week per TP. Most inbound transactions will be similar at a rate as follows 60%, 20%, 10% and then misc. For example, for one simple x12 transaction we have 90 TPs. For 90 TPs we have 1 outbound transaction script, and 8 inbound. And, actually it's one inbound script that's parameter driven. The parameters define the rules.
Also, inbound is typically more challenging that outbound. If you are returning a compliant x12 transaction, for the most part, you should be able to have one process create all outbounds the same way.