Depends if you have already maxed out 1, or at least have a competitive build for 1 mech. Build the second after you have a strong primary mech.
Building 1 good mech has taken me 3 months, with a lot of play. Once I had gone so far, I spent a little money, and went further.
This is a long term game, and requires a lot of patience with building your machines.
Have a plan and work towards it. Know what you want/need to build an awesome mech, and max out the weapons that you definitely keep. Also, don’t make silly hybrids, they don’t work, except in the case if maybe one weapon from another build type.
All the best with your mech building!