Search Engine Optimization or SEO isn't a silver bullet that you purchase once and it does the job forever.
Like any other marketing effort, it's an ongoing process. You need a plan and then you need to work your plan.
And even then you may have a business where SEO will never be very beneficial, and if that's the case, spending heavily on SEO is the equivalent of lighting your money on fire.
I suggest taking the time upfront to layout a plan, inventory the assets you have to use, think about what success would look like in a SEO strategy, and figure out how to make it part of your overall marketing strategy. And finally, set up systems so it's being monitored and executed on whether your at work or on vacation.