Every month is a bit excessive- it`s not like testing more often prevents HIV transmission.
However, testing every three to six months seems reasonable, to me. Love and trust are two of the most important things in the world, but your own well-being matters too- which is why it`s never a bad idea to consistently use condoms, even in a monogamous LTR. If you use proper protection, the odds of contracting HIV are likely smaller than the odds of having your trust breached- even if you`re entirely certain that it never would be.
If you trust your partner enough to not wear condoms, be aware that your faith in him to be monogamous doesn`t make you immune from contracting the virus. Condoms are a safer bet, for preventing transmission. As many loving, faithful, monogamous couples as there are out there who forego the use of protection because they trust each other completely, there are still others who get test results that shatter their worlds because their trust was misplaced.
I`m not saying don`t trust anyone ever, but do be aware that all it can take is one breach of trust to potentially contract something nasty.