Lust in terms of a marriage would be anything in sexual context which either objectifies, or puts forth the selfish [sexual] desires above, the spouse.
So, thinking about having sex with your spouse while at work or something would be simple if you were doing so because you wanted to satisfy your own sexual desire by “entertaining” yourself, even if only in your head, would be wrong because it is objectifying your spouse as a ‘tool’ for sex.
Now, if you were in the same position, but you were imagining having sex with your spouse because you missed them, or wanted to be close to them, and you focused on the mutual feeling of closeness with your spouse during sex, that might be a different story, because you are focusing on the unity of your marriage instead of the sex itself (and the pleasures there-in).
Now, you could still say it is wrong to fantasize about your spouse while at work because you are distracting yourself from your responsibilities, which would be unethical.
Also, I would consider it more healthy if the person were to withhold the sexual thoughts, retain the desire, and when they go home to their spouse, to “express their desire” as that promotes the unity better than just imagining their self with their spouse.