6 months sound too short for a major upgrade. Including acceptance tests, sea trials, preparations for transferring to another navy and special modifications for them and removal of some other sensitive equipments it shouldn't be less than 1 years per ship and imo won't take longer than 1.5 years if Indonesian officials do not get another party involved for the approval of the ship, and if done straight we can expect both ships to be done in 2-2.5 years.Honestly, I'm not the right person to answer since I believe @Anmdt and some others are far more knowledgeable about this.
Anyway, I'm assuming that it can take anywhere between six months to one or two years depending on the availability of the dry dock, starting condition of the ship to be refurbished, as well as the amount of equipment to be replaced. It could be less if all those things are still considered good enough and there are no need to make major modifications.
But what is more important, have parties reached an agreement on the financial plan? I doubt if the order has been put into the action,yet. Will it be at least done within 2021?