If you've only started to follow true crime recently you may not understand that the DA only gets one bite of the proverbial apple. And upon arrest, a defendant is entitled by law to all the evidence that's been gathered against him or her. Until a DA is convinced they have enough evidence to convince a jury of 12 BARD (beyond a reasonable doubt) that the defendant is guilty, it does them no good to move forward with an arrest because that starts the ball rolling and they have to deliver the goods or lose the opportunity to obtain conviction.
If a jury acquits a defendant of the charges, that person can never be charged again for the same crime by the state (hence the "one bite of the apple"). Smart DAs know that you don't rush a case just because you really want to get the guy or gal. Especially if it's someone who is not a flight risk, has ties to the community, and isn't considered a danger to the general public, then they gather the evidence and wait till there's enough in their opinion to take it to a grand jury and get an indictment and, most importantly, prove the case in a court of law, in which they hold 100% of the burden of proof.
Saying what the timeline 'should be' implies a person thinks they know more than the professional investigators doing the job. 3 months is actually relatively fast for a spousal murder case, based on various cases I've followed, some of which took years before a spouse or family member was arrested.
