Most employers want to ,a nd are happy to reimburse you. Afterall, you've given them an interest free loan, and and reliable one too. But usually more than 30 days, and over a reasonable amount you should remind them.
If your boss asks you to do x, simply say yr waiting for the previous reimbursement which is more than $100, so can you have petty cash.
Unless you have a signed agreement, they can simply decide not to. At that point you would apply that amount as an 'unreimbursed business expense' With the new tax plan in place however, Itemizing (even with a large mortgage and thousands in charitable contributions) is almost a thing of the past.
The only thinkg you can do is ask for money upfront and complain about it on glassdoor.com after you leave.