How does an Indiana trial court decide to grant spousal maintenance?

I cannot answer that question definitively. The law possesses a tantalizingly simple air: "The trial court may make an award of spousal maintenance upon the finding that a spouse’s self-supporting ability is materially impaired. Bizik v. Bizik, 753 N.E.2d 762, 768 (Ind. Ct. App. 2001) ."

Unable to give you a definitive answer, I can give you an example courtesy of the Indiana Court of Appeals' decision in FITCHPATRICK v. FITCHPATRICK (NPF opinion; October 24, 2007) at page 3-5:

2. The parties were married on March 31, 1993. There has been an irretrievable breakdown of the marriage exists [sic], and that more than sixty (60) days has elapsed since the filing of the Petition for Dissolution of Marriage on April 17, 2003.
….
4. There are no children born to the marriage for which an order is required.

5. The Wife is disabled within the meaning of the Dissolution of Marriage Act and this Court entered a final order on 10/19/04 finding and ordering that the Wife was entitled to permanent maintenance of $150.00 per week until further order of the Court which findings and order are incorporated herein.
….
8. The Husband has a vested pension benefit with Daimler Chrysler with 30 years of service as of July 2006, 10 years of which was acquired through the joint efforts of the parties, March 31, 1993 to April 17, 2003.
9. CATHY A. FITCHPATRICK has been determined by the federal government, Social Security Administration to be disabled and unable to engage in substantial work activity and currently draws the sum of $1202.00 per month less $66.60 for Medicare Premiums for a net of $1135.00 per month SSDI benefits for her disability.
10. The Court ordered CATHY A. FITCHPATRICK examined by the physician of the Respondent’s choice, Donald L. Roegner, M.D. and his report has been filed with the Court. Pursuant to the findings of Dr. Roegner, CATHY A. FITCHPATRICK “is in no position, despite multiple medications and regular treatment to be expected to work, even in the most mundane of jobs such as clerking in a convenience store, etc.”
11. The Husband is employed at Daimler Chrysler earning in excess of $26.00 per hour, approximately $1180.00 gross wages per week for a 40 hour work week.
12. The Husband has adopted M-Plan as the family health care plan and the Wife’s $1400.00 per month medications current cost $10.00 co-pay, $50.00 per month.
13. The Wife’s Medicare prescription plan would pay approximately 2 months of the $1400.00 monthly prescriptions costs of the Wife without M-Plan.
14. The Wife’s monthly prescriptions costs of the Wife without M-Plan would exceed her monthly income.
Conclusions of Law
A. The marriage is irretrievably broken and should be dissolved.
….
G. There has been no change of circumstances since the entry of the 10-19-2004 order of permanent maintenance.
H. CATHY A. FITCHPATRICK is entitled to permanent maintenance pursuant to I.C. 31-15-7-2 in that she is physically or mentally incapacitated to the extent that the ability of the incapacitated spouse to support himself or herself is materially affected.
I. The current order of $150.00 weekly maintenance is appropriate under the circumstances in that there has been no material change of circumstances since 10-19-04 justifying modification of the weekly order entered “by agreement” and may not be modified. Haville v. Haville, 787 N.E.2d 410, 417 (Ind. Ct. App. 2003), Voigt v. Voigt, 670 N.E.2d 1271, 1277 (Ind. 1996).
J. The (court) denies the Wife’s request to have the Husband pay her COBRA Insurance and maintain her on his insurance. The Court finds that the Husband is Guardian for four (4) children and such requirement would create an unreasonable burden upon him.